Recommendations for dining near S. Congress/Congress Ave?
Your best bet IMHO is a place a couple of blocks east of South Congress on Oltorf: Curra's, definitely one of the best authentic Mexican restaurants in the entire Austin area. You pretty much can't go wrong with the menu, though one thing, their Mole, that's particularly good, and actually tastes like real homemade Mole, unlike what most places serve.
Other than that, there are a lot of great places on South Congress. Woodland is getting great buzz.
Cocobean, It all depends on what you are looking for. For Mexican, many people would recommend Guero's, (and it's always hopping), but I don't think the food is very good, although I think they make a great margarita. Curra's, just a few blocks off Congress on Oltorf, is hit-or-miss, depending on who's cooking, how busy they are, and many other variables. I don't think any of their dishes are reliably excellent, although I've usually enjoyed their chile relleno. For high-end Italian, I think Vespaio is very nice. I also enjoy Woodland, which offers a slightly funkier South Austin atmosphere. If you want brunch, I'd recommend either Enoteca Vespaio (the less-formal little sister of Vespaio) or South Congress Cafe (which is a pleasant, nicely designed space). I mention brunch in particular only because I've never been there at any other time. They may have a great lunch and dinner, but I couldn't attest to that. Many people recommended the carrot cake french toast at South Congress Cafe, but I found it to be overly sweet and incredibly dense--I'm a large guy and I couldn't finish it.