I'm not a big fan of 6th Street, it reminds me of how Georgetown used to be and is very collegiate when school's in session. But if you find yourself in that area Casino El Camino is a great bar [lots of old-school punk rock on the jukebox], and they have the best burgers in town. Sometimes you have to wait about an hour for 'em, but they're huge and damn fine.
Also try Lovejoy's, which is just north of 6th on either San Jacinto or Trinity, I forget which.
The Continental Club on S. Congress is my favorite, if I could only go to one place in Austin that would be it.