Rio Grande is a good choice depending on the time, though. 4 and later it gets absolutely packed and the margaritas are pre-mixed in massive quantities and poured out of water jugs. If you go early, you won't have that problem.
The food isn't very good, but there is a Blockheads on 50th between 8th and 9th with almost all of its tables outside in the middle of a plaza. It's a great place to hang out and drink margaritas away from traffic!