For NYC:
Beauty and the Beast is a good one! (and it's a plus that Donny Osmond is in right now) It's theatre (I forget the name right now) is VERY close to Time's Square. Also, WICKED (is my favorite) and that's at the Gershwin Theatre. That show would definetly be best to buy tickets for online, but I'm not sure if there are any available right now, because it might be sold out.
You could also go to TKTS and get the 2 for 1 deal. (2 tickets for the price of one.) HAVE FUN!
EDIT: To find tickets for a show just look on TicketMaster.com