Short Day Trips:
Houstom Museum of Natural Science at the Mall, followed by a trolley ride and an ice cream cone and playing in the fountain at Mkt St.
Woody's Feed Store on Robinson Rd -- follow Woodlands Parkway EAST to dead end and then turn right and jag left over the RR tracks and go another mile or so past the first 90 degree curve and just the yellow metal church building. They have organic honey, soaps, eggs and a bunny named Jack and several horses to admire. The folks running the store are kid friendy and it's a nice 2-hours side-trip. Just watch the kids around the parking lot as large trailor rigs do back-in to pick up hay and horses, etc.
'Real' Vacations:
If you sign up for Travel Zoo's weekly specials, they have som eincredible deals to all-inclusive Mexcian resorts in Cancun/Cozumel and these place do family vacations just right. SOmeitme you can go for as little as $400 pp, kids stay/eat free.
Overnight in San Antonio - water parks, river walk, just because
Galveston - stay n a nice hotel and do a couple of morning beach walks, see a movie, eat hot dogs on the beach, go to bed early ....
Good luck and have fun!