The JW Marriott in Cancun meets all of your conditions, with the exception of the casino. It is a beautiful newer (under 3 years old) hotel, and if the boss is paying, would seem to fit your parameters. Nice restaurants, nice beach, lots to do or NOT do as you please.

I'd avoid Grand Cayman for the moment due to the devastation of the island from last summers hurricane. It is going to take a long time for this beautiful island to recover. The Marriott keeps pushing it's opening further and further back. And yes, no casinos on the island.......excellent snorkeling however.

Aruba would probably fit your plans perfectly as well. I am booked there next April but have never been before. Can only tell you that it would seem to fit into your 'wish list'.

Have fun.