Use a plastic sand pail for the container and then add some or all of the following items:

Beach Ball
Beach Towel
Blow-raft or swim ring (these could also be used for containers depending on how large you want the gift)
Water Wings
Flip-flops
Frisbee
Plastic Shovel
Sand Molds
Sunglasses
Snacks and/or Candy Bars
Suntan Lotion
Swim Goggles/Mask

You can blow up the beach ball, water wings or [...]