This is probably too late now but when I was in Cayman Island I took a pic of my feet crossed (I was bare foot though) with the ocean behind my feet. Turned out to be a cool pic.
Thought it might be cool, if you live by water,beach or something, to photograph a pair of cool flip flops or a pair of the 9 yr old's flip flops w/your digital camera in water or on the sand or something like that. Then print that up and put it on the cake.
Just brain storming...............