Congrats on your upcoming marriage.
Where to stay, where to stay . . . How long are you going to be in the Keys? How ofte do you want to pack up and move? It seems like every 30 miles or so from Key Largo to Key West has a different personality.
Key Largo has its diving, and is a popular destination. I don't go there, so I'm not much help.
Islamorada is famous for fishing and diving, has many poular destinations. I've found several spots to eat that every car I rent seems to get sucked into. Islamorada Fish Market and Lor-e-Lie to name 2.
Marathon is probably the more "quiet" of the major keys. We own a home there and love it.
And Key West is - well Key West. As they say this is where the outragous personalities turn professional.
If it were me, (and we did get married and honeymoon in KW), I'd go there, party, eat, relax, find some adventures, relax, eat, drink, party, relax, eat, find some more adventures, SCUBA, fish, tour the island, and so on.
Not necessarily in that order.
As far as where to stay, there are as many spots to choose from as your budget will allow. We stayed at the Galleon when we got married. LOVED IT. They have a pool, very small beach, Tiki Bar by the pool, it is close to Duvall, and the Marina.
There are many B&B's to choose from. I've not stayed at any of them because most of our trips to KW are day trips.
What to do? Tour the island on the trolley. Rent scooters and tour the island on your own. Stop in the cemetary and pay your respects. Go to Mallory square for the sunset festival. Tour the Key West Lighthouse, KW aquarium, Smathers Beach is a nice place to hang out. Walk up and down Duval, seek out a spot to people watch.
I think that if you tried, you could get all that in on your honeymoon, but then, you are on your honeymoon . . .