Look for good happy hours and find the drunkest hottie.
North Austin
Cougar... I mean Cool River on Thursday nights if you're into older women.
Domain has a few bars with decent talent starting thursday.
classic 6th st
Not bad for 2-minute drill on a weekend night. Easier on the pockets during weeknights with great drink specials. It's hit or miss, but sometimes you bump into a drunk cute college girl that just wants to have fun. I've also had my beer goggles on from drinking too much or playing wingman. Not much for "mingling" though.
Warehouse district
I agree with gpsss on warehouse district.
So many bars to meet pretty women. I'd have to say all the bars are good
If you like stripper or provider look, Phoenix on the weekend.
For mellow bar scene, Fado Irish bar or 209.
West 6th near Lamar
JBlack's has a lot of talent. Usually packed on the weekends but decent crowd on Wed.
Another place with some decent talent is Union Park. I like the boom boom room
Live Music like gpsss mentioned. Blues on the green during summer wed nights and it's free. Tons of festivals throughout the year. Saxon Pub or MoMo's or Continental Club.
Non-drinking spots
Town Lake trails (running).
Met a couple cuties at Butler's park (pitch n' putt).
If you have a dog, a dog park like the one off Riverside.
Austin is a great singles city.