Don't forget to visit Queen's Staircase and Fort Fincastle on your walking tour of Nassau. My son loved both. There's a wonderful view of the harbor from atop the fort.
Last year we went to Cable Beach (you don't have to pay for beach access - the resorts are MORE than happy to let you walk thru the front door and hopefully buy from the beach vendors and drink at their beach bars) and parasailed. Not for the faint of heart, but I sure am glad we did!
Cabs really are fine in both ports. They ferry about a LOT of cruise ship passengers and I've always felt perfectly safe. I too sailed with my son alone - for his 13th birthday.
We did the glass bottom boat trip and he really enjoyed it, too.
Carnival Elation March 11
Carnival Imagination Sept 07
Carniival Sensation Dec 06
RCI Sovereign of the Seas Sept 06
Carnival Miracle Sept 05
Carnival Glory Sept 04
Carnival Fantasy Jan 04