My sons birthday is in January, an indoor party for a four year old! As he is the first I totally over did the whole thing, but I got a lot of compliments and my son and his friends still talk about it.  It turned out to be a blast. Lots of planning and lots of work. But extremely cheap!!! I printed a pirate ship invitation with a treasure map (for directions) inside asking the maties to dress in jeans and white tee shirts.  I found four mattress boxes (cardboard) I drew an outline on one  of what I found for a pirate looking ship, painted water splashing on it, lined the planks on it with black marker, used a gold marker for the trimmings, and named the hull slippery shrimp. I cut out port holes.  I attached the ship to the front of my sofa with string and velcro, I made a mast of cardboard, the sail was of a plastic table cover(dollar store) hanging from the ceiling above the ship. I moved a nightstand on the side of the sofa to be used as the plank. When the "mates" arrived I had in separated bags vests I made from grocery bags, painted colorfully, I used toilet paper tubes covered with black paper as telescopes, long skinny balloons as swords, made eyepatches out of old fabric and face painted tattoos on their arms. Each child had to dress up and get on board the slippery shrimp, where I took a Polaroid picture of them peeking out of a porthole, they had to climb onto the couch and walk the length and then they had to walk the plank. I gave the photo to the parent as they left as a thank you. With one of the other boxes, I painted the inside with blue and lite blue swirls the outside with green looks like grass, looked like a pond. I drew and cut out sea critter shapes, crab, oyster, jellyfish, seahorse etc... and placed them inside the "pond" attached a paperclip to each, used a small dowel rod, attached fishing line and magnet, each child got to fish for a sea critter, and collect a prize written on the back of the critter. With leftover pieces of cardboard I cut out about 20 island shapes and painted little X's and palm trees and maps, this was for musical islands, played music, when stopped whoever is not on an island has to go to the brig, where they get a prize and a drink of grog (punch). I cut out a alligator shaped head cut and bent down teeth on top and bottom painted the inside red with tongue, used squirty foam insulation to create the form on the top of the head and painted the whole thing with green's and black it really turned out good. I attached a fishing line through the top of the mouth and strung it through a hook in the ceiling in the hallway and attached end to door knob at end of hallway. When I pulled at the end of the line the mouth went up and down like a chomping alligator, the kids had to get one of three fish (baby socks filled with beans tied at end) into the mouth as it was chomping and the got a prize. I covered two laundry baskets (square plastic) with brown paper bags put one on top of the other making a chest connected them on the back side with pipe cleaners, drew planks similar to ship on it to look like a treasure chest. I lined it with old scraps of fabric, red silks purple satins etc... and filled it with the goody bags, brown lunch bags with loot written on the outside. So at the end of the party they had to follow clues to find the treasure chest, they ripped it apart and got their goody bags.