We love to visit the Fort Worth Museum of Science and History. It is newly renovated and is great!
The school where I work does a Stockyard's field trip walking tour. It was really interesting. (The "cattle drive" takes less than 5 minutes. The cattle meander down the road.) The maze is fun in the stockyards too.
There is a Lego place opening at the end of March at Grapevine Mills. We went to Lego Land in California and this looks to be much, much smaller. It still might be fun though! Also the waterpark at the Gaylord Texan is scheduled to open around that time I think.
One of our favorite cheap things to do is to visit the Japanese Gardens ($5-6 adults, less for kids) and the botanic gardens (free). Bring quarters for the fish food machines. Seriously, my kids always have a great time when we visit this place!