I understand where you're coming from. I, too, find it hard to enjoy "beach bum" type vacations. My style of outdoors travel always involves activity-- constant seeing and doing. When I try simply to sit in a beach chair or swing in a hammock like so many other beach resort-goers enjoy, I get bored within 30 minutes!
That said, I have
learned how to slow down and enjoy do-nothing days on the beach. Yes, it really does require effort! I have to focus on letting all my other cares slip away. A couple stiff drinks helps a lot with that.

And having a book to read or some music to listen to helps shift me into relaxation mode. ...Basically it's the same combo I use on long flights!