The Denver Aquarium is loads of fun, what with sunken temples, re-created flash floods and piranha tanks, but it's also pricey. Between parking fees, multiple gift shops and added activity charges on top of already steep admission costs, a visit there can waterlog your credit card. Luckily, there's Sharkey Fun Zone, a nearly hidden indoor play area at the back of the complex (accessible via a door near the restaurant entrance) packed with giant-sized whales, sharks, octopi and other sea creatures that your kids can romp on for as long as they'd like — for free!