Lamanai is a blast. However it is a very long tour so make sure you have plenty of time to spare. We also did cave tubing there once, it was fun also. Can't help much with Roatan, I am a diver so I always dive there. You can do a discover scuba with a lot of the dive shops, this will allow you to dive without having to be certified. There is a wall right off shore, so it's a pretty easy dive.