2lbs sledge with a 6" disc of 3/8 steel welded to it. Picked up the disc at the local steel yard for $4. Took about 2 minutes to make a couple of passes with the welder. Works great for setting my sand stake, tent stakes, as well as smashing cans. I don't worry about bringing a plate for the bottom. A flat rock works fine. Even if your rock less you can usually get them pretty flat just smashing them on the ground.
Tip 2. Buy a small sledge with the plastic handle vs wood. That way the head is never loose. Drill a small hole at the base of the handle and run some 550 cord thru it. When you hit the beach set your sand stake and then clip the hammer to the carabiner. That way whenever the rest of the group wants to use your hammer "and they will" they know were it is and don't have to keep asking you.
Tip 3. Seems obvious but make sure your cans are empty. It smashes them so flat, that any contents will be upon your person if they are left in the can!