Ooooh! I'm not sure how much money you're willing to spend, but you guys should definitely check out the Spindal Top restaurant downtown. They have some amazing food, it' extremely romantic and the restaurant SPINS so you get a great view of Houston in 360 degrees!
Also, The Aquarium restaurant is really fun. It's a lot more laid back than the Spindal Top one...AND they have a ferris wheel, so you could still see the city! The inside of the place is really cool...it's a MASSIVE aquarium with all sorts of sea creatures. AND there's a touring area with lots of sea animals...and if I'm correct, they have white tigers there too!
Kemah is super fun especially with the warm weather...there's rides and restaurants and gift shops. Very fun!
Hmm what else...are the Astros playing? If there's a game, baseball games area always super fun!!
Good luck! There's LOTS to do!!