When IcelandAir launched its stopover program in 1948, the year the airline made its debut, it was out of necessity—not savvy marketing. Without a license to travel directly from North America to Europe, the only way for the carrier to fly was to offer a layover in Reykjavik. “Back then, the airline operated flights on two separate licenses,” explained Michael Raucheisen, a spokesperson for IcelandAir. “One from North America to Iceland, and another from Iceland to Europe.”

By 1955, said Raucheisen, “the thought stuck that we should let these travelers see Iceland,” and the free stopover was officially born. Being able to pit stop in Iceland for a few daysbetween connecting flights was like getting a free vacation within a vacation—it became an instant hit with hippie backpackers through the ’60s, and though it has been offered continuously since then, a concerted marketing effort a few years ago brought it back into the spotlight.

Since 2012, Iceland’s stopover program has seen year-on-year increases of 30 percent to 39 percent, with 50 percent of IcelandAir’s passengers flying via Iceland and 31 percent of that subset taking advantage of the free extended layovers, which can range from one to 13 nights. Ask any tour operator in Iceland what effect the stopovers have had on local tourism, and they’ll unanimously agree: IcelandAir’s promotion has been the single largest catalyst for the exploding Iceland tourism industry.

At least a dozen other airlines since 2012 have followed suit with similar programs. Last year, the Portuguese airline TAP added a robust stopover program with one- to three-day layovers in Lisbon, which is the second-closest European capital to North America, after Dublin. “In our 75-year history, we had essentially been selling ice to Eskimos,” said Gareth Edmondson-Jones, a TAP spokesperson, about the airline’s tendency to sell Portugal to the Portuguese diaspora. Focusing on a core group of visitors to Portugal meant planes were full in the summer and empty in the winter, and the airline was struggling to stay afloat. “A stopover program became the key to our expansion strategy—it lets us sell all of Europe to all of North America,” he added. Approximately 40,000 travelers have taken advantage of the program since its launch, and the airline is in the midst of a renaissance moment.

Since not all stopover programs are created equal, we’ve put them to the test. Here are the ones worth trying and the ones to skip—plus a few extra hacks that’ll get you two vacations for the price of one.

The Best Airline Stopover Programs

IcelandAir

Stopping in Reykjavik is convenient for many itineraries from the U.S. to Europe—particularly if you’re heading to Scandinavia. And a user-friendly tab on the airline’s homepage makes it easy to find and book stopover flights from 18 destinations in North America to 26 destinations in Europe. Not all city pairs will yield solid results, though: Route availability was limited when we plugged in a trip from New York to Barcelona, for instance, forcing us into an inconvenient (and expensive) itinerary for the sake of a weekend in Iceland.

TAP

This is one of the best deals, and not just because Portugal makes a natural stopping point en route to most European (or even African) capitals. First, the stopovers are easy to book and superflexible: They’re available in either Lisbon or Porto, for intervals of 24, 48, or 72 hours. Then there’s the companion app, which helps you get around or book partner hotels at special rates (or get a free bottle of wine at select restaurants). But the best part? Adding a stopover decreased the total cost of our flights by as much as $40.

Qatar

Book a multi-city fare with a few days in Qatar,and your total cost shouldn’t go up by more than $15—making a stopover essentially free. What you will find, however, are exclusive deals: $20 tours of the new I.M. Pei Museum of Islamic Art, dirt-cheap desert safaris, and preferential rates from five-star hotels. Plus, the airline will waive the costs of a transit visa for stays up to 96 hours.

AirCanada

Compared to some other programs, AirCanada’s stopover scheme is a limited one: It subsidizes only the cost of a hotel room in Toronto, Montreal, or Vancouver if your flight has a layover of six hours or more. (The hotel rooms are free if you fly in a premium cabin.) It’s worth it, however, to break up a long flight between, say, Houston and London—where you could pit stop in Montreal for a meal at Joe Beef and a few hours of sleep in a bed, rather than a cramped steerage seat.

Singapore

Adding a long layover in Singapore won’t come free on this airline, but your costs on the ground might as well be. Packages that start at $43 per night include everything from hotels to airport transfers—more than justifying an otherwise inconvenient promotional itinerary that nets you a cheap flight to Asia.

Etihad

It’ll cost you around $25 to add an Abu Dhabi stopover to your flight, but you’ll save what you’ve spent when you let Etihad coordinate an experience that suits the length of your layover—a steeply discounted set of holes, say, at the top-tier Yas Links Golf Club if you have only six hours to spare. Staying a little longer? They’ll set up free overnights at five-star hotels if you’re flying business or first class.

Turkish

Though you can’t stay overnight with Turkish Air’s TourIstanbul program, passengers with 6- to 24-hour layovers can join scheduled walking tours to see such sights as the Blue Mosque, Hagia Sophia, and the Grand Bazaar free of charge. Everything from the guides, to museum admissions, to airport transfers is included—and you can leave your luggage with the service desk at the airport for a minimal charge.

Stopovers to Skip
Although most stopovers are promoted as “free,” some are anything but. Finnair made a push for its new Helsinki stopover program last year, but we found that the stops were hardly free; on some routes, they added as much as $320 to a fare and took us out of our way. Ditto stopovers in Sydney with Qantas—which cost about $200 and would be helpful only for onward journeys within Australia. The worst offender was Japan. A route we tested from San Francisco to Bangkok was $2,732 without an extended layover in Tokyo; with the layover, the price jumped to $4,169.

Other stopover programs are overly cumbersome to book. To book a Panama City stopover through Copa, for instance, you have to put your airfare on hold and get on the phone with a reservations agent in the hope that your route is eligible. And Emirates makes you jump on a call with a travel agent or a local Emirates office to add stopovers to existing tickets.

Other Booking Hacks Worth Trying
Stopovers aren’t the only way to book a free (or ultra-affordable) side trip. The website Clever Layover lets you put in your origin and destination points along with a preference for length of layover—it then suggests airfares that save you money by adding an extended stopover. (Think two days in Copenhagen or Berlin on your way from New York to Vienna.)

And even when airlines don’t promote stopovers, they’re worth plugging into a multi-city airfare search. Stopping in Lima for a few days on your way to Cuzco often reduces fares on LAN, for example. (And who doesn’t want to pit stop for a couple of excellent meals in this skyrocketing culinary capital?)

For loyal fliers, United and Alaska Airlines’ reward programs actually allow for free stopovers no matter where you’re heading—as long as you’re using miles.

And if all you’re after is leisure, leisure, leisure, think about booking a cruise with Regent Seven Seas: the luxury liner actually includes pre- and post-cruise hotel stays at all its disembarkation points.

This article was provided by Bloomberg News.