The first thing we do is go to Mermaids and play the old, original, penny machines in the back. They were the first "ticket out" machines that we ever saw, so it's mainly just for memories. But Mermaids also has very fast cocktail service, if you can get past the uckiness.
There is an outdoor second-floor bar area at Fitzgerald's that's great for watching the zip line, the FSE and just people in general.
I agree with roycpa, you need to spend more than half a day downtown. We're going for 5 nights in October and 3 of them will be downtown and we can't wait!