Lots of folks bike in Texas in the summer. The trick, of course, it to go early in the day. Even in the worst part of the summer, average overnight lows in the DFW area are still in the upper 70s. If you can hit the road shortly after sunrise, you can get a couple of hours of biking in, and still get home before it hits 90.
I haven't checked it out yet, but there's a new business right by downtown FW on the Trinity River that rents kayaks and canoes.
http://www.kayakfortworth.com/
For sports, if getting over to Arlington is too much of a challenge, the Ft Worth Cats play minor league baseball at LaGrave Field, just a mile from downtown.
http://www.fwcats.com