Natural Bridge Caverns between San Antonio and San Marcos is pretty popular. Tubing is also super popular in the New Braunfels area. It's not my thing, but there are a number of options and the weather is still usually warm enough to do this in October. You'll also be before Wurstfest but after summer crowds, so it shouldn't be too crazy either. Alternately, there's the original Schlitterbahn, where about half of the park uses water from the Comal River. If you want to see the bats in Austin, you might look into some of the dining or cruise options since the crowds and parking can be madness. Beyond that, it's fun to walk around UT and get that Keep Austin Weird vibe along with all of the UT stuff (since school will be in session at that time for fall semester). Oh...and if you're a Chuy's fan, you can dine at the original Chuy's on Barton Springs Rd. Speaking of, if you're into popular food places...there are all kinds of options between both cities (too many to mention them all), but Lulu's bakery in SA for the giant cinnamon roll and getting in line at the crack of dawn to get BBQ at Franklin BBQ (Austin). Oh, I also like walking into the Menger Hotel in SA to get that old SA vibe when visiting (and hope we don't spot any ghosts LOL). Another vote for the Buckhorn in SA. Oh, and it's kind of cheesy, but for whatever reason...we get a kick out of going to the Magic Time Machine restaurant in SA.