What day of the week are you coming into the city? If it's a Wednesday, Saturday, or a Sunday and you have any interest in the theater you can hit an afternoon performance. There are two places that sell half price tickets the day of the show, one at Times Square, the other down at the World Trade Center, it's a decent way to get into a show for a decent price. Ask if you are interested, the people here are extreamly helpful. We were at a play last night in the second row and the people next to us were there on half price tickets they got that day (no not us, full price), so you can get some very decent seats.

The least expensive way into the city is to take the shutle bus over to the Howard Beach subway station, then take the subway into the city. It's $1.50 per trip, or as many others have said, $4 for an all day pass. Each person with you will need a pass, you can't just give it to the person behind you and have them go through after you.

If the weather is good the Circle line does a 90 minute trip around southern Manhattan and the Statue of Liberty, it's a great way to relax, and it's half the length of the full round the island trip.