Plan your child's 12th birthday on the beach or at least have a beach party if you cannot have the party on an actual beach. Play games with the party guests that give them the beach feeling. There are plenty of traditional party games that you can alter to a beach theme that are age-appropriate for 12 year olds.
Purchase some sand dollars that are all approximately the same size. You will need ten of them. Paint five red and five blue before the party and allow them to dry. Use a piece of poster board to make a tic-tac-toe board. You could also use foam board if you want something a little bit heavier. When it is time for the game, explain the instructions to the guests. Set the game board on a table and separate the group into two teams. Do not let guests see the game board once play begins until they have taken their own turn. Blindfold one person from the red team and allow him to place a red sand dollar on the board. He may then look at the board. Blindfold one member of the blue team and allow her to place a blue sand dollar on the board. The first team to get three in a row wins.
Buy some play sand if your party is at home. Use beach sand if you are at the beach. Fill two play buckets with sand and put a plastic spoon in each bucket. Divide the guests up into two teams. Place the buckets of sand at a starting point and an empty jar at another point. Guests must use the spoon to fill the jars with sand. They cannot use their other hand to help hold sand. If your jars are especially large, you may want to mark off the fill line with a piece of tape. The first team to fill their jar to the fill line wins.
Set up a volleyball net in your backyard or at the beach. At home you can use a regular beach ball, but at the beach you may need a regular volleyball, as the wind may not make it possible to play with a beach ball. Just allow the kids to have fun hitting the ball back and forth. This game does not need a lot of supervision for the 12 year olds. This gives you an opportunity to get food ready or prepare for the next game. This is also a good game to play at the beginning of the party when guests are arriving or at the end while guests are waiting for parents.