Grrrr... I had a BIG post with lots of links... and lost it!
I'll try again...
in Waco, there are several things that might interest you... the Dr. Pepper Museum and the Texas Ranger's Museum....
http://www.wacocvb.com/waco-attractions.asp
Lots to do!
In Austin, the botanical gardens is in Zilker park, and there is also a very cold spring fed pool there... Barton Springs pool
Austin also has the Bob Bullock Museum....
http://www.thestoryoftexas.com/the_museum/about.html
Georgetown has Inner Space Caverns....
http://www.myinnerspacecavern.com/home.php
Another cavern in the area (San Antonio) is Natural Bridge Caverns....
http://www.naturalbridgecaverns.com/history.aspx
In San Antonio, along with the Alamo, are several other missions... really neat to see!
http://www.visitsanantonio.com/visitors/play/attraction-d...
San Antonio also has a bunch of museums...
http://www.visitsanantonio.com/visitors/play/history-heri...
I went to the Institute of Texan Cultures with my daughter's 7tbh grade class long ago... really nice!
http://www.visitsanantonio.com/visitors/play/attraction-d...
This last link lists a lot of free or lower cost items....
http://www.visitsanantonio.com/visitors/play/san-antonio-...
Have fun!