Search the forum, I've suggested stuff in the past and can't remember it all right now.
Car stuff - come downtown and check out the classic car club. If it's nice out, like it's expected to be tomorrow, the doors will be open and they're friendly folk. A lot of the cars will be out for the weekend, but there's always something interesting there. Last time I stopped by their mechanic was wrenching on their E-type...
There's the Ferrari dealer on Park Ave in the 50s.
Beer - The Gingerman is centrally located but can be a very annoying atmosphere. Tons and tons of stuff on tap though. I've heard good things about Rattle and Hum, but never been. Avoid all Heartlands at al costs. In West Village there's The Blind Tiger and Vol De Nuit. Both get very crowded but have great beer. Blind Tiger is more American microbrews, Vol specializes in Belgians.
If you're willing to go out to Brooklyn (and you should) and suffer the f'in hipsters, the Brooklyn Brewery has weekend open houses that are fun, lots of good stuff on tap. Tour is OK. Also nearby are greats like Spuyten Duyvil (belgian), Radegast Beer Hall (never been), Mugs Ale House (not bad), and d.b.a (which was great the one time I went... heard mixed things since).
Off the beaten path stuff that isn't beer and cars - come down to Chinatown on Saturday or Sunday morning and do dim sum for brunch. Golden Unicorn is probably your best bet for a good balance of non-Chinese friendly and authentic. Get there no later than 11am and try as many plates as you can. Bring a bunch of friends. Share a table with a local and strike up a conversation, it'll be great.
If you're at all interested in donuts, try the Donut Plant on the Lower East Side. Get there early and expect a line, but tasty, imaginative donuts.
Walk the High Line. It'll get crowded since it's going to be a nice weekend, so either endure that or go early. Dusk seems to be a popular time to visit.
Are you a runner? If you like running and beer, learn about "hashing" or "the hash house harriers" then meet up with the crowd on Sunday. Check out hash nyc for specifics. I used to be a regular with this group, haven't been in a while.
Try and get lost in Central Park. Stay away from the touristy southern end and go north. It's a great park.
If you're going to the top of a building to see the view, I suggest Rockefeller Plaza over the ESB. Easier lines, great views. Also, you actually get to look at the ESB, which is very interesting. Very nice at night.
Basically - stay off 5th ave, stay outta Times Square, Little Italy, and Herald Square. If you're in the Village, sure you can walk down Bleecker St, but wander off it a lot, the area is awesome and Bleecker is just one dinky little street that runs through it. If you go to SoHo, stay the F off Broadway. It's not SoHo, though Old Navy, Banana Republic, and the likes would like you to think it. Also, stay away from the Rock and Roll Hall of Fame Annex, which is nearby.
I travel with my stomach and suggest you do the same. There are a ton of great restaurants in every cuisine and price range here, eat at a different one each meal. I will personally find you and cock-punch you if you eat at McDonalds, Burger King, Ruby Tuesdays, Outback, or any place you could find at home. Don't bother with "NY Pizza", it's just pizza. If you're inebriated, try one of the Papaya dog places: Gray's Papaya, Papaya Dog, Papaya King, etc. Good drunk food, as are the many falafel/shwarma places.
Anything else?