My five year old nephew probably couldn't cave tube but my 9 year old niece could.
Just take them to the playground. Give them some buckets and shovels. Give them some snorkel gear. Have them bring down some cheap plastic floating mattresses, floating rings(tubes) and then when they leave they can give them to some local kids. Have some beach balls around for tossing and kicking.
The nephew just got back from a family vacation in Mexico. He went for a long walk down the beach with grandpa every day. They would climb some rocks too. He loved playing in the pool, having his face painted and organized games and activites for youngsters.
His little cousin (same age) can't get enough of riding around in (or driving) his grandma and grandpa's golf cart.
Make sure there's lots, and lots of juice available.
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A fish and a bird can fall in love, but where will they build their nest?