The River Market is pretty nice on the weekends. Not sure about the weekdays. Check out Cache Restaurant. They are upscale and sort of pricey but the food is great.
Willie D's and Ernie Biggs are dueling piano bars but with you being 18 I'm not sure you can actually get in. All of my "fun" comes from bars. When I was 18 the most I did was sneak into movies and hang out with skaters.
You can get into Gusano's if you're under 21. They should be pretty busy with it being a big football weekend. And you can get into the Flying Saucer until 8pm but they will make you leave after then.