Earn Your Scales
Create a small group of swimmers. Tell them every time they swim from the shallow end to the deep end and back, they will earn a scale. When the whole group has 50 scales (you may want to change the number based on ability level), they will get to have a mermaid makeup party. At the party provide wigs with different-colored long hair, blue and green eye shadow with sparkles, fake eyelashes, lipstick and blush. Buy mermail tails where dress-up clothes are sold or make tails out of shiny material or aluminum foil. Let the children work on their own makeup and on each other. Serve fried shrimp or fish sticks for lunch.
Ugly Mermaid Contest
Not all mermaids were considered pretty. Have an ugly mermaid contest where girls use all the makeup and wigs to transform themselves into hags. Take closeup shots of the hags and post their pictures on a board. Let everyone vote on the ugliest mermaid ever to swim the seven seas. The winner should get a funny prize like a ribbon made out of shells and a bouquet of seaweed.
Mermaid Trivia Contest
Watch a mermaid movie and test the girls' memory by asking them trivia questions. Reward a girl with a shell when she gives a correct answer. At the end of the game, the one who has the most shells gets a real mermaid makeover by a local artisit or a theater makeup artist. Take a picture of the finished mermaid and put it in a frame.