If it's not already all booked, Apple Camp at The Woodlands Mall. It's free and they teach the kids how to make a movie using ipads and garage band software. After the movies are made you go back for the Film Premiere to see everyone's movie - walk the red carpet and all. Pretty fun for 8 and up.
Explore the tunnels downtown and end up at Discovery Green for a cool splash off at the splash pad.
Get out and go geocaching. Lots of fun and plenty right around here to find. Houston area is loaded with them. Free app and kids have fun finding them. It's a cool treasure hunt.
Sea Discovery Center down in Lake Jackson. It's free and something different to get out to do. Town Center in Pearland has a small splashpad for a stop on the way back.
Go visit the "Car Art Museum" and do a drive by at the "Beer Can House". Both are free and different.
Go tour Mrs. Bairds Bakery. You have to register in advance but the tour is interesting and you get all the hot buttered bread you want at the end.
Tour the Nuclear Power Plant down in Bay City if you have kids interested in science for sure. it's free too