S.B.
The zoo may be a compromise. There are many indoor exhibits. And if you haven't seen the MOLA (Museum of LIving Art) it's pretty neat.
It's in Dallas, but the Dallas CHildren's Aquarium is reasonable and great for kids ($8 for adult and $6 for kids. Feeding the sting rays costs $2). It's on the small side, but if you take your time, you can make an afternoon of it. It's right behind the Dallas Science Museum, you could easily do both places in one afternoon. The Dallas Museum has a terrific children's museum!
It's in Dallas and I think it is very expensive, but the Dallas World Aquarium is great!
In Fort Worth, we have a Science Museum as well. I really consider this more for Children, but it would be a fun place to go play. They have Grossology going on right now...all about the gross stuff your body does. It's a pretty fun exhibit. The new Risk exhibit is geared towards older kids, but even my three year old got a kick out of laying on a bed of nails. The Cowgirl Museum is right next door. We have never been, but my out of town company always wants to do something with a cowboy theme. If you don't want to do the stockyards, this museum may be an option.
The Bureau of Engraving and Printing is in Ft Worth. It's one of only two locations where money is printed. It's free and indoors. Just visit the website before making plans. They are really strict on what they allow in the building.
I think the Cowboys stadium does tours. If you have sports fans coming, that might be fun to check out. But I have heard it LOTS of walking. It may not be a great one for really young children or anyone with mobility issues.
We have lots of museums...the Kimble Art Museum, the Amon CArter, the Sid Richardson, the Texas Civil War Museum (my Dad is a Civil War buff and loved it, my kids were bored), Stockyard Museum, and the C.R Smith (sometimes they have TERRIFIC kid exhibits).
Then there are places like the Main Event around town...bowling, video games and lazer tag. And Arlington has the rock wall climbing place, but the name escapes me right now.
Hope this Helps!