Crissy listed some pretty good spots...I suggest you check them out.
Depending on your taste in nightlife, you may want to avoid the 6th St. area...it's largely the kiddie crowd. All the bars/clubs are pretty much interchangeable during the weekend...but during the week, a few places (Friends, BD Riley's) have some really good live acts playing, usually for no cover. If you're a blues fan, find Joe Richardson and/or Alan Haynes...you won't be sorry.
The grown-ups tend to frequent the 4th and 2nd St. areas west of Congress. There's a pretty cool open patio/bar atop The Speakeasy, and Belmont has a good vibe to it, too. Cuba Libre is another cool spot, so long as you like rum. Another good lounge place is Molotov...a couple blocks west on 6th.
This time of year, I also recommend you check out Barton Springs pool. The spring water is constantly in the low 70's, which feels damn good when it's over 100F in the shade.
Just saw your percentages...if you want to check out the gay scene downtown, head to Oil Can Harry's or the Rainbow Cattle Company.