Cheap is always relative. I'll start off with some not-so-cheap ones, but that are less than $200/day.
Schlitterbahn - Water Park. Very nice and located about 1-1.5 hours south of Austin.
Fiesta Texas - Six Flags theme park in San Antonio. You missed AstroWorld in Houston, they've closed that down. I think there is still another Six Flags up in Dallas.
Sea World - Sea World in San Antonio.
I've been to all of the above, and they're great. They're more fun if you can convice a few friends to go with you.
Tubing - I've not done this yet, but I'm sure you can look up details on this. I hear of folks doing this all the time. Just put on the sunscreen so you can be tanned for your first day of classes instead of lobster red.
Boating - There are plenty of lakes around on which people boat. The only problem is finding or knowing someone who owns a boat. Or you can find someone willing to blow their first year's student loans buying one.
Zilker Botanical Gardens - Free entrance. It is just a set of walkways with various plants and displays near the downtown Austin area. You won't spend a whole day, but it is a nice couple of hours.
Otherwise there are the old stand-by generics of computer games, movie marathon (you could spend a whole day each on LotR, Star Wars, or Star Trek).