For These European Sights, You Should Reserve Tickets Ahead

Because travel has become so affordable, the world's favorite tourist attractions have never been more crowded. Some of them are so overrun that you could spend as many hours outside in the queue as you'll spend seeing what's inside. But thankfully, the Internet has transformed your ability to book tickets ahead of time.

Obviously, you should always plan ahead for popular restaurants, temporary exhibitions, most walking tours, and experiences you have your heart set on. But when it comes to major, world-famous attractions, some of them are no longer very easy to get into if you haven't made advance plans. Even if you buy a museum pass that grants entry to places around town, you still may have to wait in line if you don't have a timed reservation. So before you board your plane to Europe—in fact, when you buy your airfare—make sure you have your tickets in hand for these attractions or they may be sold out when you get there.

The Louvre—and slip in using the entrance inside the Palais Royal–Musée du Louvre Métro stop. If you forgot to book ahead, get your ticket from the Civette du Carrousel tobacconist in the Carrousel du Louvre shopping center (99 rue de Rivoli), where there’s rarely a line. Then enter via the Métro stop or the Porte des Lions in the Denon wing.

The Eiffel Tower (pictured)—book as early as you can, and don't forget your dining reservations in one of the two high-quality, sit-down restaurants, if you want them.

The Catacombs—the 1.25-mile underground route can be tight, so walk-up visitors may have to wait, but if you have a timed reservation, you're in.

Cooking classes at Le Cordon Bleu—because the one-day courses offered in English always fill up months ahead

The Anne Frank House (pictured)—which got so crowded that now it only sells timed tickets, released two months ahead online, with a precious few more released on the day

Van Gogh Museum—it has also shifted to an all-timed ticketing system, sold ahead online, so it's a bad idea to show up without a reservation.

Leonardo da Vinci's The Last Supper—found at the church of Santa Maria delle Grazie; slots sell out weeks ahead.

La Scala—last-minute seats for opera and ballet usually aren't very good, and operas sell out months in advance; plan ahead if you want to see the likes of dancer Roberto Bolle (pictured).

The Uffizi (pictured)—your life will be much simpler if you avoid the dreadful line outside by booking ahead.

Michelangelo's "David"—he's on display at the Galleria dell’Accademia, where there's usually a hefty line that smart prebookers can skip.

The Leaning Tower of Pisa—anyone can see it from the outside and take a goofy picture, but if you want to climb the tower, book online weeks ahead.

The Colosseum—you could wait in line, but why waste the extra time?

Galleria Borghese—the spectacular art museum where only 360 visitors are allowed at a time on the ground floor, and all of them got in with advance-purchase tickets

The Vatican Museums & the Sistine Chapel (pictured)—ironically for a holy place, the line can be hellish, but to have advance reservations is divine.

Sagrada Família—whereas it's free to see from outside, you haven't seen anything (or learned about Gaudí's vision) unless you've purchased a timed ticket to get inside (pictured); access to the towers requires scheduling, too.

Park Güell—another part of the city's Gaudí mania; make plans in advance to see this complex and its museum about the architect.

Palau de la Música Catalana—either for tours or for performances, you'll need advance tickets.

Picasso Museum—although it's free on Thursday afternoons, for saner crowds, it's smart to get timed tickets ahead of time.

The Alhambra and Generalife—lives up to its origins as a fortress by repelling all without reservations, which are available starting three months ahead and usually sell out.

The London Eye—where you'll still have to wait in line to board, but at least you'll skip the other line to buy tickets, plus you can choose a sunset slot.

The Houses of Parliament—closures are varied and many, so make sure you've nailed down your entry weeks before.

The Making of Harry Potter—carve 4–5 hours out of a day and book as far ahead as you can to tour the actual sets where the movies were filmed (pictured).

Buckingham Palace—the doors are only open for two months a year; if you want to go inside, you'd better get on that.