The Texas Ren Fest is the largest Renaissance Fair in the US, and it is glorious. It's about an hour north of the city, and you should buy your tickets online. We are partial to the Birds of Prey, Fire Whip, and Mud Show performances. And it is worth it to stand in line for a chimney cake and, of course, kettle corn.
Houston has a fantastic museum scene, and the venue is located right next to it. The Natural Science Museum and the Museum of Fine Arts are both huge and phenomenal, and the Houston Zoo is a fun time right next to them. We also have a great Children's Museum and Health Museum, both of which Regan loved as a kid. Even closer to the venue is the Menil Collection and Rothko Chapel, both we would highly recommend.
It's NASA! What more do we need to say? The Space Center exhibits themselves are great and have a lot to offer for both adults and children. If you go, you definitely should do a tram tour. You can see the Mission Control used during the Apollo moon landing! Definitely book your tickets in advance. This is about 45 min - 1 hour south of the city.