An alternative to the Circle Line is taking the free ferry to Staten Island, around sunset. There's going to be plenty of people, but if you get there in time you can get yourself a place outside, and the view of the sun setting is really awesome. Also, on coming back to Manhattan (once you get to Staten Island, you need to get off the ferry and then you can get on it again straightaway) the view of Donwtown approaching and of Brooklyn on your right is really beautiful.
Central Park is very nice and it can be romantic, just choose a slightly secluded path if you want it very romantic - avoid the Meadow, the large lawn close to 54th st, coz that's normally packed
It's true that the Empire State building can become very romantic at sunset, yet the long line can be seriously unpleasant!
Time Out is a mine of brilliant ideas, it lists ALL events, it gives tips on bars, restaurants and stuff. Also, you can enter a bookshop - say, any Barnes & Nobles... the one on Fifth Av. is huge! - and pick the 'Zagat survey' guides to hang-outs, restaurants and stuff (they also have a website, you can check that out too). They normally list a lot of very nice places with reasonably cheap prices!
If you want to treat you to a night on Broadway, you may wanna consider 'Rent': they do a ballot for every show... if you win it - you just have to show up at the theater in advance, on the very day of the performance - you can get tickets for the first two rows for as little as 20 bucks. It may not be easy, but if you live in NY, you can try again and again, it's worth it
Have fun!! I wish NY weren't on the wrong shore of the Atlantic for me...