Two of the best museums in the world want you to pay admission upwards of $20, and many of us do.

Further, when we buy those tickets online - $16 per adult for the American Museum of Natural History, $20 per adult for the Metropolitan Museum of Art - we pay an extra $4 service fee. Per ticket. Though after the first admission, the Met knocks the service fee down to $2.50 per ticket.

All that said, if you decide to wait until you get to the museums to pay, you'll notice something unusual. That $16 and $20 are recommended admissions amounts. But once you step up to the admission desks at these decidedly intimidating places, you may ask yourself, is it really acceptable to pay less?

According to staffers at both museums, you better believe it. Since well before the economic downturn, many visitors to these museums have paid only what they’re comfortable paying.

Not all tourist attractions are this flexible, of course. But there’s no reason why you should pay more than you’re comfortable paying, at least some of the time.

Keep an eye out for free days.

Other major museums, as well as zoos, gardens, and parks worldwide set aside free, discounted, or pay-as-you-wish admission days or times. The trick is tracking them and planning your life around them. The Thursday or Friday sections of daily newspapers in your destination “often list what's going on, and a freebie section can usually be found” within that, notes Paula Rivera, manager of public affairs for Hertz, who notes that most of these daily papers as well as bargain-filled local weeklies “have also migrated over to the Web, thus making [the free days and times] easier to find.”

And if you’re looking for bargains on a more national scale, keep in mind that 147 of the 391 National Parks waived their admission fees on three weekends this summer, the last of which is August 15-16, points out Jon Douglas, managing editor of Bing Travel.

Seek out discounts in things you already bought.

If you’re considering European train travel and a rail pass purchase, “don't pay for any museum admissions until you make sure they're not free or greatly reduced when you present your rail pass,” says tour leader and Trip Chicksco-owner Ann Lombardi. For instance, in Switzerland, the price of a rail pass includes free admission to more than 400 museums, Lombardi says, “along with free usage of most lake steamers and postal buses, which whisk locals, tourists, and the mail to the more remote villages. “

Douglas notes that “membership to your local aquarium or museum often gives you free or discounted entrance at similar institutions around the world.” If you belong to your local children’s museum, check its Web site as well as the Association of Children’s Museums for an idea of how far you can stretch your membership dollar.

Also keep admissions savings in mind when you’re booking airfares and hotels. “I love hunting for hotel-themed packages that include passes to local museums and attractions, says Traveling Mamas blogger Beth Blair. “These specials save travelers from having to seek out the passes while saving them a few bucks and maybe letting them score a meal or cocktail, too.” Also conduct your search from the other direction: If there’s a particular sight on your must-see list, scope out the hotels in the area first, particularly the mid-range chains whose promotions almost always include free admissions to nearby attractions.

This spring, Scandinavian Airlines offered a Copenhagen Card to anyone booking a round trip fare to Copenhagen, recalls travel writer Lisa Davis, a managing editor at First Class Flyer. The card “grants free admission for 24 hours to 62 museums and sites, including Tivoli Gardens, and also gives discounts to select restaurants and car rental companies.“ Subscribe to your favorite airlines’ e-bulletins to stay on top of promotions, Davis advises.

Go for bulk.

If you happen to miss out on airline promotions for multi-site passes like the Copenhagen Card, it might pay to buy them anyway, as long as you do a fair amount of sightseeing. Douglas points to a handy combo pass in Rome for the Colosseum, Palatine Hill, and the Roman Forum that “allows you to bypass waiting at all three sites, while the Roma Archaeological Pass offers the same privilege for nine archaeological sites in the Eternal City.” He and many of the other experts asked also sing the praises of the multi-site Citypass, available in many major cities.

Speaking of bulk, you know those brick-like Entertainment coupon books that you pick at throughout the year for discounts in your home city? Buying one for your week-long vacation might pay off, too. “There are over 150 editions, including Orlando, Las Vegas, Hawaii, San Diego, Atlanta, and New York City,” says Suzanne Rowan Kelleher, co-founder and editor-in-chief of family travel site WeJustGotBack.com. “Titles cost between $25 and $50, which you could recoup in just a day or two. On the last day of vacation, I like to leave the book behind for an incoming family or as a tip for my hotel's housekeeper.” She adds if you sign up for the book’s e-mail alerts, you can “find out when titles go on sale for as little as $10, so you can pre-purchase for future trips.”

Scour travel-oriented associations.

Rivera advocates AAA membership for practical purposes but also points out that the association’s “Show Your Card and Save” program yields discounts to many attractions and sites, and Kelleher notes this extends to theme park tickets, vacation packages, and other discounts.

Your destination’s Convention and Visitors Bureau (CVB) is also a fertile place for discovering local admissions and transportation discounts, Blair and Lombardi say. Kelleher advises checking the CVB’s Web site for “hot deals” coupon pages and “two-for-one tickets for major attractions.” When you get to your destination, spend a few minutes in a branch of the local visitor’s bureau, where you’ll find additional coupons and often unpublished insider tips from staffers.

Try the off season approach.

The off-season has long been a budget traveler's dream, Blair points out. “In the mountains where people can enjoy summer and winter, off-season is actually ‘mud season’ -- after the ski resorts close --and is the perfect time to explore the mountain towns with cheaper rates and lack of crowds.”

Speaking of skiing, if you want an "economical" way to experience the 2010 Winter Olympics for a fraction of the price, Davis says, consider swinging by the Winter Dew Tour event in Breckenridge, Colorado, “where Olympic athletes competing at Whistler in 2010 will come and ski and do demos, she says. “You can meet the Olympians and watch them ski for free before they head to the Olympics.” She also points out that the event’s dates, Dec. 18 – 20, is Breckenridge's value season, “so room and condo rates and lift tickets are also discounted.”

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