Bourne Lake is a good place near San Antonio to do some open water swimming. They don’t allow any motorized boats on the lake, so you don’t have to worry about getting hit by a boat while you’re swimming. San Antonio has many running and bike trails that you can take. I included a few links below. Let me know what day you plan on being here. I’ll keep my eye out for any group runs or bike rides that you may be able to join in on.
it's not tri related, but take a Segway tour if y'all have time. my wife and I did that in September 2014 and it was pretty cool. we didn't get a chance to workout while we there.