I've only been to Belize out of your list and I would recommend doing some kind of excursion. Snorkeling is very good there. At the pier area are some shops, mostly the touristy type, but the areas around there are beautiful and worth doing a trip to see.
We have been to all three. SAMANA was our favorite although it was
a number of years ago. There were people that wore actual grass skirts.
It was very primitive but extremely friendly and nice.
Roatan - We took an excursion up a mountain and round one turn were stopped by a number of soldiers with guns. They boarded the bus, exchanged shouts with the tour guide and left us after a few minutes.
Didn't like it.
Belize - took a very nice boat ride to some ruins. Took the entire day but had agreat time.
We have been to Belize and Roatan twice. Try the cave tubing in Belize with Cavetubing.com and ask to have Richard. The port is very nice and their bus picks you up right there. You have a 45 minute walk through the jungle (carrying your tube, but Richard is willing to carry anybody's who needs help) and it is just amazing, then you tube through two caves and end up where you began the hike. They have a guard to watch your valuables on the bus while tubing. Floating down the river in the jungle of Belize is an unforgetable experience!! We went on the zip line in Roatan which is another experience I will never forget since I was talked into it and I know now what jelly legs feel like. We rented a driver, Victor, for the day in Roatan for $25.00 a person and he took us to the zipline, to see the monkeys (free, which a tour was around $45) and to a wonderful beach where you could get $5 massages! Both places are wonderful for making memories!!
We did the zipline in Roatan and went with the dolphins. Great time. In Belize the highlights were the cave tubing adventure and decided to do the zipline there as well - great, and liked the rainforest and limestone cliffs, which gave it a plus over Roatan's zipline. Went with Major Tom and the Fun Team - this is not an airhead, screaming "caaaaave tuuuubing" bunch who chase away all the wildlife with their insipid screaming through the rainforest, and blocking out all sound inside the caves (which idiot would scream INSIDE a cave? With stalactites hanging overhead????), but we were with a group of highly trained, very very knowleadgable guides working under their head, Major Tom, a genuine army major with an incredible wealth of information, and his son, Minor Tom, who is so funny and totally friendly - made us feel like family rather than tourists - and unlike the other bunch we read about, NEVER once tried to grill us for tips - they were genuinly surprised and very happy when we tipped them -and accepted the tips very shyly and were very sweet about it. Total safety is obviously their top priority. They also had better caving lights than everybody else in the caves. You have got to ask for George of the Jungle, Lover Boy Mike, Minor Tom - that trio are totally hilarious - we had a party coming back on the bus - dancing, cold beers, you name it. This is a first class, very professional operation (and we gather Major Tom put it all together 22 years ago, and actually organized cave tubing at the park back in 1992) - other senior guides at the park vouched for the Major - and obviously deeply respected him. You can catch pics of this man and his operation at www.cave-tubing.net or the sister site www.belizecavetubing.net. Oh - and do try the rock jumping - awesome.