10-12K? No way would I waste that on Atlantis, but I am not a fan of all inclusive.
How much vacation time does he have?
We did 2 weeks in France for 5k when DD1 was almost 2. Don't be intimidated by the plane ride.
I would probably do something in Europe or an extra long Disney World/Disney Cruise trip or rent an awesome house for a few weeks in one of my favorite beach towns (Cape May, NJ) and go crazy w/ eating out/spending money. Maybe a few week trip up the coast of California into the PNW? Rent a car, stay at awesome hotels?
Those are my initial ideas. I would take at least 2 weeks for sure.
Have you asked what kind of taxes he needs to pay on this?