I just bring a little mini safe that holds cards and phones, then I carry around the key on my wrist. The safe attaches to a beach chair, so I suppose someone could cut it, but it would be pretty obvious. I also use it on the ship when we're in the pool.
Additionally, when packing, I fill it full of odds and ends in my suitcase so it doesn't just take up space, and frankly helps me keep track of small items.