Some easy ways to be active at the beach:
Surfing/ body surfing
Walking in the water knee/waist deep
Get up off your chair/ blanket often and walk down to the water
Dig a whole/ build a sand castle
Picking up seashells and sea-glass
Flying a kite
Some not-so-easy, but fun ways to keep moving:
For some bodyweight exercises that are less discreet, but great to do at the beach, check out Movement is Medicine: 5 Bodyweight Exercises You Can do Anywhere.
Don't forget the sunscreen, and always stay hydrated! Check out of my favorite kind of sun protection (hypoallergenic, anti-aging, all natural, never tested on animals) here!