When the best travel plan is no plan

It was, clearly, the perfect layover. But for me, it was more than that—it also was a throwback to the self-assured bohemian I once was, who would spontaneously meet up with Danish backpackers on an island lake in Sumatra, or cycle through Vietnam with little more than a backpack and a compass.

I had lost that girl somewhere over the decades, buried beneath color-coded vacation calendars, pooled mileage points and prepaid excursions, all carefully curated months in advance. Now, relaxing on a Thai beach with my old friend, I found myself reconnecting with that 20-something adventurer, open to all possibilities, unfazed by the unknown.

That was when I made a promise to myself: Once my kids were launched, I’d pack that spontaneous, no-plans version of me on most of my travels. I would pick the destination in advance and book the flights, but other than the first night of a hotel and a rough sketch of what I didn’t want to miss, I would leave much of the day-to-day to the fates.

In a quantified world where we optimize everything on a spreadsheet before taking a risk, this surrender to a destination has felt like freedom, a rebellion against an algorithmic, quantifiable, approach to travel. My serendipitous sojourns since then haven’t always worked out perfectly, but I am getting better at accepting providence as my tour guide. All it takes is a willingness to relinquish rigidity, the confidence to chat people up, and a phone full of apps.

A guy in a pickup

The first time I and my husband, Colin, tried this approach was a few summers ago in the Brazilian capital of Brasília. We quickly discovered that foreigners need a guide to enter many of the buildings, some of which are open to ticket-holding outsiders only on certain days. Unfortunately, this insight occurred as we were standing beneath the blazing sun on a Tuesday outside the Palacio Planalto, which houses the president’s office and is open to guided groups only on Sundays.

A google search connected us with a dubious dude in a pickup truck who told us that his father had whisked him from California to Brazil decades earlier during a contentious divorce. He knew everyone (cops, ice-cream sellers, homeless people, government workers) and dashed us into the Planalto, the Palacio Itamaraty and other architectural marvels we thought were off-limits. He regaled us with all sorts of barely believable tales, after which we had lunch with his Brazilian wife in the employee cafeteria of her office, which happened to be in the stylish headquarters of the attorney general.

The next day, we taxied to the National Congress, but it was in session, so closed to travelers. We explained to an empathetic security guard how desperate we were for a peek at its retro-futuristic midcentury modern architecture; she offered to stealthily show us around during her break. She also drove us to the stunning, swooping Alvorada Palace and secured a tour slot (at the time available only on Wednesdays from 3-5 p.m.) through a friend.

Winging it, we told each other, had led us to more access, not less. We were convinced that we were onto something.

So in Mexico City, we pre-booked our hotel in Condesa, and that was all. Then upon arrival, I texted a Mexican hotelier I had met in Chicago, who invited us to a party for surrealist artist Pedro Friedeberg. That led to a dinner invitation the following night at the home of some artsy locals. Hardly an evening we could have scheduled! (Fortunately, I had packed some nice outfits; a must for serendipity travel.)

In Panama City, we went a step further, and didn’t even book a hotel for our two-night stay en route to the eco-retreat, Islas Secas. We searched in the taxi for a place to stay and secured a killer suite for less than $300 at the American Trade Hotel, a restored neoclassical jewel from 1917 smack in the heart of the old town. Hot and hungry, we scuttled over to Fonda Lo Que Hay, hailed as one of the best restaurants in Panama, and scored the last two seats at the bar. Chef José Olmedo Carles Rojas even chatted us up and revealed his favorite outdoor markets. I was smitten—and this was before he opened his hot spot in Los Angeles.

It doesn’t always work out

Of course, to paraphrase Walt Whitman, wending to shores I know not without an itinerary doesn’t always lead to captivating interactions and discoveries. On a recent trip to Boulder, Colo., the hotel I prepaid only as my plane descended wasn’t just under construction, it was also hosting a rollicking (read: all night) revival meeting for teens. My door had a gaping hole big enough for a human to slither beneath.

An autumn odyssey from London to Chicago required a touchdown in Boston, so we made a little road trip out of it, bookended by dinner with friends in Boston and a visit to our daughter in Connecticut. We hopped the ferry to Cape Cod, but, it turned out, wandering around the peninsula without a car, in the rain, wasn’t the best way to see much of anything.

Or consider a comparative experiment we conducted between two trips to Paris—one planned, the other freewheeling.

For the planned trip, we booked a room at the fabulous Plaza Athénée, set up dinners with friends, and got timestamped tickets to a Van Gogh exhibition at the Musée d’Orsay and a blockbuster Mark Rothko retrospective at the Fondation Louis Vuitton.

For the spontaneous adventure, we didn’t book a thing—and ended up sleeping in a pretty gnarly one-star hotel. I tried to fill unscheduled hours with vintage-clothing shopping and did talk my way into an exhibit at the Pompidou Center, but heading up to our sad temporary residence each night dampened any small thrills I had managed to sprinkle into the day.

The lesson: Sometimes unplanning works beautifully (see: Brasília). And sometimes, it’s not so great. But the highs you get from serendipity are so great they are worth the occasional bust.

Another takeaway: An itinerary can be a good servant but a hard master. Once you’ve written it and committed to it, you’re basically left to check off the to-do list you’ve prepared for yourself. That’s perfect if you want to see and do specific things, but it’s important to remember that the original point of travel was to open the mind. Marco Polo didn’t know what he would find when he set out! But when all the discovery happens at your desk while you’re doing research and booking timed appointments, there is little room for surprise or chance.

The art is in the balance—planning for a few things I don’t want to miss, and letting fate take over the rest. Generally, as long as I don’t arrive with a strict agenda, unplanning works wonderfully. Upon arrival, my Instagram algorithm automatically changes and some cool regional stuff pops up, which I immediately populate into a google map, or at least “save" into a file. I’m a big fan of hyperlocal social-media accounts, such as “Secret Paris," which unearth fantastic finds that I’d likely never come across on my own.

I love how unscripted escapes force me to actually engage with locals. My go-to insiders? Women in effortlessly fashionable outfits; tell her you love her shoes, then ask if she knows of a great restaurant nearby. Chat up the waiters, who know where the action is late at night. Facebook friends can also be unsuspecting insider guides, and in my experience, they enjoy hearing from a ghost from the past seeking their wisdom. Sometimes they even meet you and bring martinis!

Where planning still pays off

When does spontaneous travel not work? Off-the-beaten-path destinations. I can’t imagine having tried to “unplan" the trips we took as a family to Oman or China, where we really needed a wall-to-wall itinerary and often, a native speaker as a guide. If I’m unlikely to be returning in this lifetime, I’d like to suck the marrow of the place on my one visit. I’d never try to unplan a trip to a resort destination, either. Chances are slim that a sun-dappled room will appear at the right price.

Similarly, your travel companion makes a difference. I’d only attempt serendipitous travel with a partner who’s unperturbed by ambiguity. So if it’s just me or me and my husband traveling, we might figure it out on the fly. Kids joining? We likely need to plan because they are grown-ups and have school and jobs and friends to get back to. Plus, I learned, if I don’t entice them with some exciting activities, they simply won’t carve out the time, even if I block a week in their calendars.

Still, those are the exceptions. The more I invoke that fearless backpacker from college, the more comfortable with uncertainty I have become. I’m a rebel fighting against the modern urge to max every experience, to tick the boxes of some made-up bucket list.

The serendipity mindset has even started to creep into my daily life at home. I used to check my calendar and to-do list for the week ahead, and panic that the former was too empty and the latter too full. Now, I just assume that those unbooked hours will fill on their own—whether by an impromptu invitation, a leisurely walk around an undiscovered neighborhood, or an unanticipated project.

I actually may be getting worse at planning trips, or planning in general. But so what? As long as I get my job done and come through for friends and family, no one cares how packed or lazy or spontaneous my travels and my life are except me. So I’m embracing the unexpected, the unforeseen, the uncertainty. And as a result, to quote the inimitable Stevie Nicks, I’m back to the gypsy that I was.

Heidi Mitchell is a writer in Chicago and London. She can be reached at reports@wsj.com.