Matt_Little said:The characters will between levels 3 through 5 to reduce the chance of easy escape. If they do manage to find some easy way off the island such as a spell or magic item I didn't plan for, I'll simply not allow to work and claim it was something on the island.
Remember that this is D&D, not television. If they find a clever way to get off the island that you didn't plan for, then it should work.