Park at the Port Authority Bus Terminal. Look it up. Basically you just drive up the ramp the buses do when they come in from NJ, but just veer off about half way up to the parking garage. You can leave the car there and take your keys with you. Plenty of parking. Just check the pricing if you come and go through out the day. I think max is like $28 per day for one longer period.
Bonus is that it's five minute from Time Square and if you're leaving Manhattan by going west, then the ramp dumps right into your own lane for the Lincoln Tunnel; no traffic!
It's Funseeker! I'm all about the good times!