Collect some cool shells, beach glass or driftwood. Use a broken shell with a sharp edge to write your name or message in the sand. Decorate your scene with the shells, etc. Take oodles of pictures. Try different angles and different distances.
Print your picture out and put it in a cute frame.
I debated whether or not to add shells and sea glass to the outside of the frame. The kids voted and we decided to add them with hot glue (or you could use E-6000) to the frame.
Then hang your sign on a door or set it up on your desk. You be the boss!