The Moody Gardens and the Flight Museum are awesome, if you like WW-II airplanes the Flight Museum could be the highlite of your trip, WOW they have some incredible planes.
Galveston isn't far from Houston proper at all. As mentioned before, NASA is in Clear Lake, drive on in to Houston and you can visit the Battleship Texas / San Jacinto Monument, also there is a great natural history / science museum. For the womenfolk who like to shop, Houston has a Galleria shopping center full of all the ritzy stuff that such places have. Houston traffic really sucks at the rush hour times though, plan ahead.
Out on the west side of Houston on I-10, there is a Holiday World RV dealership that is one of the very few in the Southern states that carries Arctic Fox truck campers, 5vers and trailers. Maybe not a "normal" vacation destination but fun to look at if you like the RV scene.