Go-Karts: LINE GO-KARTS OFF IH 35N @ 210-646-9941.
I would say Roof that meeting at the River Walk might be the best bet.
Plenty of food and bring up and down both sides of the riverwalk.
If you already have a hotel room down there, you don't have have to crash.
Here is a Map of the River Walk with it's surroundings
http://www.urheremaps.com/preview.shtml
Hope this helps!!