Depends on what you want to do? Do you want to see Christmas decorations?
On my list as a New Yorker I haven't done is to take a bus up to Dyker Heights to see the decorations there.
If you like other things Christmas there are some fun Christmas bars around the city. Rolf's is worth a stop in for a drink (don't eat there). Also, Miracle on 9th st is always fun (they have had a few locations as of late). And Boilermaker turns into Sippin Santa's surf shack is great!
Non Christmas the Time out Market in dumbo worth visiting. The Morgan Library is a favorite of mine where I take people who are coming into the city. Also, the High Line/Hudson Yards are worth a trip (Little Spain!)
Let me know if there was something more specific you were looking for!