My boys' birthdays are really close together, so we always do a double party. This year, they turned 8 and 4, they wanted to do a Scooby Doo mystery theme at our house.    Invitations: I went simple and used store bought ones. Decorations: We painted an old school bus shaped play hut to look like the mystery machine (this doubled as something for the kids to do). We had bought several stuffed Scooby Doos at a thrift store over the last few months, as well as a couple of Mystery Machine toys, which we set around the house for decorations. One of the stuffed Scoobys was huge (almost as big as my 4-year-old); we put him on a high shelf and put a sheet over him to use as a ghost decoration. We also turned a bunch of other little toys (U.B. Funkeys) into ghost by putting a tissue over them and lined them up on the window sills. My kids colored a few Scooby Doo coloring pages and hung those on the wall as well. Activities/Games:  I had put out a few coloring sheets and turned on a Scooby Doo DVD for while we were waiting for everyone to show up. Once everyone had arrived, we had some pizza and juice, and then tried to get all the kids' attention by showing them our great cake - only, the cake was missing. There was a note with a clue inside the cake pan that told the kids that the birthday phantom had stolen our cake.  There were also puzzle pieces in the cake pan that they then had to put together for the next clue (I had bought a package of blank puzzle pieces at an Arts & Crafts store, which we had painted and written a clue on - all the pieces had the exact same shape, so it was pretty difficult to put together).  The clue on the puzzle led them outside, where they had to collect Mystery Machines (I had found Mystery Machine shaped Easter eggs" before Easter).  Inside the Mystery machines were the words for the next clue - they were on bone-shaped cardboard - and each one held 2 or 3 words.  The kids then had to figure out how to put these words together correctly for the next clue (since we had younger kids as well I wrote numbers on the back of the cardboard so they would just have to figure out to put those in number order).  This clue led them to the "ghost stomp".  I had blown up white balloons drawn on ghost faces and stuck a piece of paper with a secret code clue in one the secret code key in another and several pieces of paper that said "not a clue"; I dropped them down from upstairs as they had finished putting the previous clue together.  They had to pop the balloons to get the clue then figure out the clue which led them back outside to "drown the mummy".  For that game I had put a clue in a plastic case the kids have wrapped up the case with toilet paper to look kind of like a mummy and the kids then proceeded to throw water balloons all over the mummy (and each other of course) until the toilet paper was washed off. When the opened the case they found the next clue - again in secret code this time mirror writing.  This clue led them to the "biggest ghost" (the giant stuffed Scooby Doo ghost up on the shelf).  They pulled on the sheet to unmask him (knocking him off in the process) and found a cake pan.  However this was still not quite the end since this cake pan was empty except for a note/the last clue in which Scooby admitted to being the birthday phantom and felt bad that he ate the birthday boys' cake so he claimed to have baked cupcakes to be found in a place that was cold.  So when the opened the refrigerator they finally found the cake! Cake:  I baked cupcakes put them as close together as I could and frosted them with green frosting so that it looked like a real cake.  I sprinkled crunched up M&Ms on for a road put one of the Mystery Machine Easter "eggs" on the road and put the whole Scooby Doo gang on top of the cake as well (luckily the Easter bunny had brought the figures just in time for the birthdays). I also made ghost lollipop by putting a tissue over a lollipop and drawing on a scary ghost face which I stuck in there as well.  Party Snacks: pizza juice boxes and watermelon Favors:  Each guest received a goody bag (bags were storebought) containing a Scooby Doo pencil Scooby Doo eraser Scooby Doo lollipop Scooby Doo Fruit Snacks and a Mystery Machine Easter "egg"."