You're going to Cozumel twice?
I'll assume one of them is Cayman - it often is.
Roatan: the complex at Mahogany Bay is nice, the beach is free, but it's all enclosed. The only locals you're going to see there are the ones wearing clip-on badges because they work there. Pffft. Take a taxi to Coxen Hole and go to their indoor flea market. If it can be made out of mahogany, bloodwood, and other native hardwoods, they've made it and they'll sell it to you ... even solid mahogany entry doors ... which they'll ship to you. Or you can buy some trivets which will fit in your suitcase.
Belize: ALWAYS get off the ship in Belize. Take five minutes and wander through the strip mall in The Zone - it won't take longer than that, then walk out the back gate onto Fort St [?]. There's a tiny Jade Shop [and Museum - the Museum is, like, two shelves in the back corner] which has a small selection of local and Indochinese jade doodads. Get something inexpensive for a christmas present, and ask for change in Belizi dollars. This shop used to be a Recommended, but didn't have enough money to bribe the local Tourism Ministry officials when they erected the fence around The Zone ... hence they are outside now.
Then cross the street and hit their flea market - much of the same stuff out here as in the strip mall, but it's cheaper and many will barter with you. There's a barbeque pit-thing there. Get a Belikan beer, and save some room for a traditional Belizi lunch of stewed chicken, red beans/rice, and fried plantain. Don't leave off the fried plantain - it's much better than it looks. If you're daring, try their habañero sauce. The orange one is cooked with carrot instead of tomato; carrot is Caribbean style.
You can hire a horse/carriage here for a quick tour, or a cab for a faster tour, or get your hair braided, or get a coconut freshly whacked open for you out here. Or you can head back into The Zone for another Belikan at the Drunken Monkey or the Loopy Lizard, or whatever it's called. Can't miss it.
Cozumel: a great place to do nothing much. One of our favorite places to start at one end of Melgar, and walk back down the street. Los Cincos Soles for vanilla - you'll probably want a full liter, and you have $15 for it. Sample the xtabentun. If you want an American lunch along the lines of Applebees and other chain bars, try Señor Frogs, Tres Friends, Margaritaville, Hard Rock, etc; if you want an Americanized Mexican lunch, Pancho's Backyard or Las Palmeras is popular, if you want a semi-authentic Mexican lunch try the Viva Mexico Cantina [upstairs, outside, there'll probably be some guy out front handing out business cards for a free tequila shot - their menu prices are in pesos so don't have a coronary; just divide by 10]; and if you want a real authentic Mexican lunch go a few blocks back from Melgar and hit any number of virtually nameless storefront cantinas with three tables each and five total chairs. They probably won't speak too good Engles, so bone up on your Españolish - caliente means hot as in boiling water; picante means hot as in spicy. Don't ask for caliente salsa, because they'll look puzzled and bring you cooked sauce trying to give you what you want; ask for picante salsa. And back here away from the restaurants on Melgar, drink only bottled beer or bottled pop.
And don't forget the vanilla. Los Cincos Soles
Cayman: Another good place to do nothing much, but it'll be much more expensive. And on the wrong side of the street, so watch when crossing. Two blocks back from the jewelry stores they want you to visit are all the banks you've heard about where greedy, rich Americans keep their money safe from greedy, rich American theives, pardon, politicians. Beyond that are the Radio Shacks and other local shopping. That's interesting to see. All the locals out here will be nattily dressed in business suits as they head off to work in the banks. There a stamp collectors shop upstairs in the tourist area which is fascinating if you're a philatelist or numismatist. And around here somewhere is a second story bar - it's blue is about all I remember - with good rum drinks, Caybrew beer, and a full menu with conch fritters. It's pricey and you can't divide by 10. If you haven't had conch fritters, I recommend them. I don't actually like them unlike the Belizi lunch I mentioned, but I recommend them just because it's one thing you've got to try while you have the opportunity. Get one order and split them three ways; you can always get more.
SIG: If you're going to create the drama, at least supply the popcorn.