Under the Sea Party for a 4 yr old.  I just threw the party yesterday and it was such a blast, especially planning for this one!  My husband has a large 220 gallon salt water aquarium "reef tank" with Nemo and Marlin in it, plus a bunch of other fish and corals so we've been into fish for a long time.  For decorations I put up light green and dark green "algae" streamers hanging full lenght in bunches from the ceilings so it felt like you were swimming through it. I added dark blue, light blue, and some orange streamers in all the doorways and across windows and along walls.  We had 2 sets of fish lights that I ordered from oriental trading company which were really cute and the kids loved them, we hung those about the room too. I got a fish net from the local party store and hung it in the corner of the room.  I hung a couple small paper fishes from it and placed a few large blow-up nemo's inside the net. I had a dozen blow-up Nemo fishes (sort of like beach balls) that we had scattered throughout the house. The kids really enjoyed playing with them during the party.  I got a huge box from the Harley-Davidson dealership for free (motorycycles come in it), that's about 7x4x4 and we put craft paper on the front of it.  I cut a door with a round hole in the door like you see on boats, and a few extra windows in the box for the kids to peek in and out of. I drew a seascape on it, and the words DORY'S HOUSE over the doorway, which my kids requested. THe house was blue.  I put out some fish foam stickers for the kids to decorate.  We were going to color it with crayons but decided it was too much.  It would have been great for a smaller party and I think we will still color it maybe when a few friends come over.  That was the biggest hit of the party.  For a craft, we created little water bottle "aquariums". I made individual sandwich bags for each child filled with fish confetti, a few little sponge sea life capsules that dissolve and you get a surprise sea life figure, and one super growy sea life that the kids can watch grow and fill the bottle in a few days.  We tested one out the week before and we were fascinated by it.  I added some baby oil to the bottle to give a bubbly effect in the water, and taped the lid shut with some clear packing tape. For the finaly activity my husband pulled some hermit crabs, snails, algae and kelp, and a clamshell out of his fishtank into a shallow container with saltwater in it to show the kids up close and that they could touch. All the kids just loved seeing the hermit crabs crawl around on his hand.  We had a small flashlight to shine on the animals while my husband told the children about them. For the food we served seashell pasta with shrimp, fish sticks, I scooped out a watermelon and cut a scalloped edge on top and used it as a bowl to hold fruit salad, blue jello in kid sized cups, 7-layer dip, chips, and sandwiches. (the party was at 3:30 and we had way to much food for that time of day!) I made a coral reef cake which I am rather proud of!  I iced it light blue, and piped dark blue around the edges.  With white chocolate (wiltons brand is fine, although I use guittard compound coating which tastes better. I melted it in the microwave and seperated it into 3 different bowls.  I added pink to one, green candy color to the one, and purple candy color to the last bowl.  I piped it out onto parchment paper to look like corals and algae (very simple) and let it set in the freezer for a couple minutes.  You can do this a week before decorating the cake if you store it in a cool place in an airtight container.  On the day of the party, I added the brightly colored chocolate corals on top and all around the cake so that they stuck up out of the "water" and had a nice 3-D look.  Then we used skewers to add plastic nemo figures (my daughter happened to recieve a whole set of all the nemo characters for her bday the week before) all over the cake. Turned out way better than I thought and the kids loved eating the chocolate decorations. The goody bags contained a large fish pencil keeper, a pencil, a jelly pen (with sea life inside the jelly part), a sea-life pencil sharpener/eraser/stamp combo, and a noodle worm ball yoyo - all from oriental trading company. I made chocolate nemo suckers, and added in some nemo fruit snacks to finish them off!