For a great little mermaid/under the sea party start with streamers blue and green draped across your ceiling like waves. I used the sticky gum stuff that you get with the glow in the dark stars.  Then get some fun foam and create fish shapes and decorate them with glitter, fabric paint and wiggly eyes and hang them between the waves on your ceiling with fish line  and use the glow in the dark stars to attach them to the ceiling. For the palm tree go to a carpet store and get the cardboard tubes the have for the trunks of your trees. You can shape thin "dry cleaner" hangers into leaf shapes and cover them with green fabric just hot glue the edges and use the rounded part of the hanger to attach it to the trunk. Use Styrofoam or another craft foam put it inside the tube and hot glue the hangers.  It really creates a neat look.  We played pin the lips on Ariel We just made her out of poster board. We played freeze dance to the soundtrack from the little !mermaid.  We had peanut butter sandwiches cut into fish shapes with cookie cutters. I made green Tulle skirts for all the girls and seashell "bras" from felt tied with ribbon and they all had Ariel party hats that we put red tulle coming out the top.  It was a very cute party.