My friend who has done this itinerary says the Ocean to Ocean excursion on a ship is something everyone should do. I've also heard good things about the railroad in Panama, but you miss a lot of the Canal since it goes inland.
In Limon, I have no idea what to do, but we thought we'd do the Jungle Cruise.
Coz - Might do the Manta Catamaran that goes to Palancar & Columbia Reefs.
Gr. Cayman - Capt. Bryan's cat. with snorkeling at Stringray City, etc.
Have you heard of anything else?