Do you mean Top of the Rock?

Plan on spending a few hours walking up and down 5th ave and looking at the windows of Saks and other department stores. They're always decorated for the holidays.

Check out FAO Schwartz and take your wife skating at Wollman Rink in Central Park.

I also like the rotating bar on top of the Marriott Marquis. Yes, it's over-priced and tacky, but sometimes that can fun. And the views are amazing - especially if it's a clear night.

Finally, I'd book a hotel in manhattan. I know you want to save money, but if it's your first time in the city, then stay in the city. The view will matter a lot less than the experience of waking up and being in the middle of the sights/sounds/smells of New York.

Consider making a reservation somewhere now (with a cancellation policy of 48 hours) and keep checking priceline, etc. until you find something decent for less money.