Regarding Lamani, everyone on our tour wore sneakers or low cut hiking boots. Some of the paths were rocky and if you want to climb up to the top of the ruins, sneakers might work better than Teva's. Be sure to bring binoculars so you can see the monkeys and birds that your tour guide points out.
6 days is definately not long enough to see Belize and AC. I stayed on AC, but wish I had more time on the mainland. Next time I will stay on the mainland for the first few days of the trip for a chance to see more of the wildlife and rain forest.
I am not a strong swimmer, so I told the guide this. He tied an orange life vest around my waist and this worked out perfectly!
Enjoy your trip!