We had a combined party for my 40th and DD's 6th birthday. She wanted an Ariel party, so we went with an underwater theme. DH did all the decorating and hung inflatable fish from the ceiling, put a reefscapes video on a TV to look like an aquarium, hung up blue lights, and stuck fish decals on all the windows. We had Ariel come to the party and DD and all the girls were so excited. My sister joked that the dads loved the mermaid too. ;)
I wore my jellyfish dress that I got from eBay and DD wore an Ariel costume. We had lots of fun!