Kemah Boardwalk is a great day trip from Houston for groups and families. It takes about 45 minutes (20 miles) to drive there from downtown. You'll find amusement park-style entertainment, lots of restaurants and shops, and a seaside resort. It's a bit touristy, but excusable for an excursion to the Texas coast. Don't expect crystal blue water; this is the Gulf, after all. The boardwalk is also a short drive from the Johnson Space Center, so you could plan to do both in one outing.
Photo via Flickr user roy.luck