Just about anything in NYC can be done in an 8-hour period. And a lot is within walking distance from TImes Square. Both the Empire State Building and 30 Rock are in the area, and you can go to the top of both. I actually like Top of the Rock at 30 Rock better, because you get a great view of the Empire State Building and Central Park. Both cost about $18-20 to go up, but it is worth it! You can spend a ton of time just walking around Times Square itself. It's one of my favorite places to just hang out - lots of shopping in and around - Macy's, Virgin Records Megastore, huge Toys R Us, M&Ms and Hereshey's stores, Apple store, huge World of Disney store - all close by. Madison Square Garden is right there You can also get a 1-day subway pass that will get you all over - and to then ends of Manhattan Island where you can see other stuff - Ground Zero, Statue of Liberty, and other stuff that would take you forever to walk to, but takes no time to get there on the subway.
There is a tourism office in Times Square where you can get all kinds of info - 810 Seventh Ave. (bet. 52nd and 53rd Sts.)
NYC is actually a great city to go to with no actual plan other than to just go with the flow - you will find plenty to do!