There's only two things I wish I had taken but didn't:
1. Binoculars. For all the reasons mentioned previously.
2. High powered magnets. These would have been so handy!
Secure your towel to a deck chair
...or, since the walls/ceiling of cabins tend to be metal, use to hang up swimsuits to dry
...won't come off the outside of the door if someone accidentally brushes up against it
...with a little imagination can do quite a bit to entertain the kids (and adults too) in case of rainy weather....
... best of all, they take up no room at all!