In Belize do a ships excursion, it's a tender port and takes another 20-30 minutes to get to shore there don't want to miss your ship. Cave tubing is an excellent choice for the family, so is snorkeling one of the cay's (where all the beautiful beaches are they are not in Belize city) if you like animals they have a zoo there or the Baboon sanctuary (really Howler monkey's)If you like Mayan ruins like I do the ruins are excellent I went to Xuanantunich one, and I've heard great views of other ruins that include the River Wallace. I would not recomend the city tour or just going shopping here at the visitors center.
In Costa Maya that's an easy cheap beach day. Walk through the cruise ship visitors center ask for the buses to Majahal town. Takes a few dollars and a few minutes there you can find a spot on the beach, eat, swim, shop, drink at one of the bars, if you want get an inexpensive beach massage.
In Rotan I've only done a ships excursion to Parrot Bay resort didn't care for it too American for me, I want to experiance the real Roatan. Next trip I'm looking for Victor Bodden tours google him.
In Cozumel grab a taxi to Paradise beach it's beautiful, you still have to pay to use the beaches here, share a cab to save $$, or take a snorkel tour we found one right off the pier only cost $40 and was better then most of the ships snorkels I have been on. The guide would dive down and bring up starfish and sea horses for us to look at, our glass bottom boat was small so our group was smaller makes for better snorkeling. Hope this helps.