The zoo and Hermann park (by the zoo) are great places to take the family for a day or just go and relax. The weather is going to be great this weekend, if I can finish some projects around the house I think I'll head down there.
Its partially closed for the season, but to go. Think "Redneck Disneyland" because most of their attractions are made from wood and pvc. but you can buy your own pumpkin for Halloween in their gigantic pumpkin patch. I'm heading there today because they sell Christmas trees as well.
also, +1 on the childrens museum; they went thru a big expansion about a year ago and almost doubled their size. i think tuesday? (check around) is free admission night.
The art car parade is a great once-a-year family thing. For something more frequent, there is an annex to the Natural Science museum down in Sugarland. It's a drive (for me, at least), but it's never crowded so the kids really enjoy it.