Travel destinations that are slightly cooler than the Caribbean?

My family just went to the Cayman Islands, which was such a lovely place! Literally the only thing I wish we could change is the temperature—we have very small children and it got a bit too hot for them during the middle of the day. They got too sweaty and cranky to be outdoors for a few hours each day. The mornings and evenings were great temp-wise! Anyone have recommendations for places that run a little cooler/shadier but have the same casual and beachy vibe? Maybe somewhere in California? We are based in the US btw—thanks in advance!