Hi bride2be417, welcome to Cruise-Chat. I sent you a PM about moving your post here, since it had nothing to do with the cruise line. There are tons of posts here about both ports. I'm not sure what your interests are, but here are some suggestions.
In Key West, take the conch train or go to Mallory Square, then walk the downtown area. If you like key lime pie, visit the Blonde Giraffe for the best key lime pie, period. In Cozumel, there is tons to do. There's the dolphin swim at Chankanaab Park, or just going to Chankanaab Park. Paradise Beach is another good beach area. There's shopping, Carlos 'n Charlies, Senior Frogs, Margaritaville, and other fun places for a couple of drinks. There are quite a few good snorkeling spots in Cozumel, too.
Best wishes for your wedding and upcoming cruise!