Agree with what others have said. SATN is fun and convenient, but not a destination trail. The 'Nut is lots of fun, and last time I went there about a year ago it had lots of new fun flowy trails with plenty of berms. But I will not go there on the weekend. Fuck that.
To me, the quintessential Austin trail is the Barton Creek Greenbelt (BCGB). And more specifically, the back trails as wd40 alluded to. However if you've never ridden it you absolutely need a guide. I have seen some halfway decent maps at Bicycle Sport Shop, but I'd say you still need someone with you who knows it.
I find myself going out of town to ride most of the time if I'm not riding SATN or BCGB. If you have some time to head a bit out of town to ride, consider Pace Bend. It's a fun trail with some climbing but not much, and some technical features to keep you honest. Plus, Lake Travis.