It you want educational:
Dallas/Ft. Worth (if you are doing the JFK stuff)(never been to any of these, just ideas):
(Ross) Perot Museum of Nature and Science - Seems to be popular
Dallas Aquarium - Ft. Worth has the Zoo, Dallas has the Aquarium
Dallas Arboretum - Girls like flowers, and 66% of your troupe will be girls (The Arboretum in Houston was neat when I went during some festival, years ago)
Ft. Worth Zoo - As others have said, I hear it is one of the best in the nation
Austin Area:
LBJ's library in Austin - It has a museum in it, but it probably wont take more than a few hours
LBJ Ranch (Texas White House) out in Stonewall - The ranch is kinda neat, doesn't take too long, and you drive through the hill country to get to it
If you want entertainment/outdoors:
Dallas/Fort Worth:
Reunion Tower - You can go up in the big ball in the middle of downtown Dallas
Stockyards - as previously mentioned. Note: make sure you look up what times things are happening to get the best experience
Food:
Babe's Fried Chicken - if there was one place you have to eat in DFW, it's Babes. They are world renowned. It is
BYOB and you get a choice of Chicken or Chicken Fried Steak
Fogo De Chao $$$ - Brazilian Steak House - Some of the best meat I have ever had
Bob's Steakhouse $$$ - Top End Steak - never been, but it's the premier steakhouse in DFW
Del Frisco's $$$ - Alternative high end restaurant in Dallas that is highly recommended
Christina's Fine Mexican (TexMex, mostly) - reasonably priced and very good
Pappasito's Cantina (TexMex) - "reasonably" priced Texas TexMex chain - best steak Fajitas I have ever had, and they know it (and price it accordingly)
Austin Area/Hill Country:
Lake Travis - Rent a boat and tour the rocky cliff shores
Schlitterbahn - never been, but you can probably tell from this thread, people seem to love the waterpark in New Braunfels
Fredericksburg - never been, but it is another popular hill country location. Some traditional German stuff, wineries, and places to hike. Also next to LBJ Ranch.
Food:
The Oasis - "reasonably" priced, awesome view of Lake Travis with average food
Rudy's BBQ - affordable Texas chain of gas stations and cooked on the spot BBQ, only the ones in and around Austin are good (I loved Rudy's BBQ when I lived there)
Franklin's BBQ - won a bunch of BBQ awards, legendary lines to get in
Pappasito's Cantina - if you missed it in Dallas, there are plenty of them in Austin, did I mention the Fajitas?
Honorable mentions:
Canton, TX 1 hour east of Dallas. The first Monday of the month they have a bunch of knick-knack vendors (~5,000) show up and sell shit. Women seem to like it.
Scarborough Renaissance Festival Ends on Memorial day, if you are traveling that early. (Went to the Houston one back in high school, was cool then, dunno how things are now)
If you want a more scenic drive from Fort Worth to Austin, you can jump over to 281. I pulled a trailer that couldn't go over ~60 mph down there once, on 281, and it was more interesting than 35 (google street view makes it easy to see if it something you want to do). Marble Falls area was also the best looking country I have seen in Texas, yet.
If you are renting cars, an alternative could be Amtrak. They offers a route from DFW to Austin in 5 hours. The time I took it, it was comfortable, relaxing, and they have cars with big windows in to watch the steers go by.