I have a 4 yr. old little girl and a 2 yr. old little boy whose birthdays are 3 days apart.  Because of that I combined their birthday parties together and did an Under the Sea/Little Mermaid theme. I made the invitations by cutting out tropical fish shapes out of colored construction paper folded in half.  I trimmed the fins in glitter paints to add sparkle. On the front I put Come take a trip and dive under the sea at a birthday party for _______ (my children's names rhyme with sea) and put all the information on the inside.  To decorate the dinning room and den area, I use brown poster board to make the bottom of a boat and an anchor and hung them on the ceiling in my den. I looped and alternated dark blue, light blue, green and purple crepe paper across the ceiling in my den and living room. I printed and cut out pictures of sea life and painted glitter on them and hung them with fishing line through out the house. I also bought gray poster board and out coral reef out lines and pasted sea plants, shells and tropical fish that I had printed on my computer onto them and placed them on the lower portion of the wall all the way around in my dinning room. (That is were the party was mainly held)  I bought Mylar balloons in the shapes of dolphins and fish and had them tied to the chairs. I had blue table clothes on the tables and for snacks I made Jell-O Jigglers and used sea life cookie cutters to cut them. I also had gold fish crackers; sea shaped fruit cocktail, sea creature fruit bars, shark attack fruit chews, and a Little Mermaid cake.  For the activities, I had all the kids go into the kitchen as they arrived and they made sea shell wave bottles. (Fill the bottle half full of water, add food color, add the miniature seashells and glitter and then fill the rest of the bottle with baby oil. Hot glue the tops on and the kids will have a blast.)  After the kids were done with the bottles I began the treasure hunt. I had taken pictures of different places in the house and pasted them onto pieces of construction paper. The children followed the pictures to the next picture until they came to the shelf were our large fish tank is.  I hung a blue/green tissue paper grass skirt on the shelf and behind it was the chest. I made the chest out of foil and gems. I had filled the chest up with sand and placed large exotic seashells inside.  I painted some of the shells with glitter and glued magnets on each shell so the children could place them on their refrigerators at home. The kids loved reaching into the sand and finding shells. After presents and cake the children got to fish for their goodie bags.  I bought small colored paper bags with stiff handles on them. I printed fish and shellfish onto label paper from my computer and painted them with glitter paints and then cut them out and placed them on each side of the bags.  I placed the bags in the kiddy pool on the back porch and had the kids use my kids play fishing poles to fish for their bags. It was hilarious. We had so much fun I didn't want to clean up. Everyone was amazed.