Make sure you can take the aircraft out of US territory ie: The Bahama`s the majority wont let you, and if you do it anyway the fine on return is in the order of $10,000 just for you and the operator has a bigger one he will pass to you.
There is an outfit in Tamiami called Dean Aviation who will let you take out certain 172`s, I did the trip to Nassau a couple of years ago it was brilliant.
Dont let the old chestnut of not having the correct visa to return to USA put you off.
The 90 day visa waiver was ok prior 9/11, I presume its the same now.