Michele, my kids will be 4 and 7 when we cruise.
We're booking everything through Disney.
You're right, we're limited because of DD's age (4) but in Cozumel, we plan to do the Chankanaab Sea Lion Discovery (can't wait to see that park)!
In GC, going to do the Stingray City Observatory and Island Tour. DD would be too afraid to do a dolphin excursion or stingray excursion where she's in the water with them.