If you plan on visiting attractions like the zoo and Space Center, definitely go in for a CityPass. It makes admission more affordable.
Buffalo Bayou is the perfect place to relax, hike, see public art, and kayak. It has a hidden secret: overlooking the bayou on the Preston Street Bridge is an unmarked red button. Push it, and you'll unleash the "Big Bubble." No worries, it's perfectly safe (and encouraged).