Yeah, no one goes to the Flats anymore unless they're either 14-18 year old girls or 45+ year old men. No offense to anyone in either demographic. Actually, that's really just the East Bank. The West Bank has Rock Bottom Brewery which has a pretty cool happy hour and the Improv, along with Howl at the Moon and a few other places. Those are all in the Nautica complex.
W. 6th (aka. the Warehouse District) is now the place to be. The Blind Pig is my personal favorite - no dress code, no cover, always packed. The other clubs on the street can be cool, but cost more and can be pretty snotty.
Depending on when you're in town, go out to Cedar Point if you like roller coasters. It's well worth the trip - and any Clevelander knows that we claim it as part of the city even though it's over an hour away.
Whitey's Booze and Burgers is another good trip to make - that's about 30 minutes south of downtown.
Personally, my favorite thing to do in the city is go tailgate at Browns games. It's INSANE, but so much fun, whether you're going to the game or not.
If you have any specific questions, drop me a PM, or I can give you my e-mail address. I've been here my whole life, so I'd like to think I know something about the city.