For my son's 6th birthday we planned a Scooby Party.  Invites we bought at the local party store.  As kids arrived we were watching an old scooby doo movie (just in case some kids had never seen scooby or the gang).  After the last guest arrived and the movie completed we had all the kids go to the kitchen to have Scooby tatoos put on.  While the kids were busy Dad removed the presents and left a bone shaped clue to find the missing presents.  After tatoos,  I directed the kids back to the great room to open presents.  They found the clue (GO TO THE PLACE THAT ONE WOULD EAT AND MAKE YOURSELF A SCOOBY TREAT) and the fun started.  I had purchased items to dress them all up like the gang.  A scooby mask, Long metallic hair for Shaggy,  a bandana tied like a scarf for Fred,  A headband for Daphne, and a pair of black sunglasses with the lens popped out for Velma.  Each child got to blindly choose from a girl dressup bag or boy dressup bag (to save arguments).  The first clue led them back to the kitchen for scooby snack.  Large bowl field with different cereals, m&m's, goldfish, etc. ( the second clue was hidden in the bowl of food).  I let each child use a scoop to fill up their own zip lock bag with scooby treat to take home.  The child that found the clue, (GO TO WHERE XXXX SLEEPS AND FIND OUT WHERE THE CREEPER CREEPS) got a prize and we were of to the next place).  In my son's bedroom we had set up a game we had bought at the local party store.  Stand up Scooby face with hole for mouth with pieces of pizza, sandwich, pickels, ice cream, etc. that each child had to try to throw into scooby's mouth.  Another bone with clue mix in with pieces of food.  (GO TO WHERE YOU PARK THE CAR,  I KNOW THE CREEPER CAN"T BE FAR)  When the kids got to the garage, there was a pinata to break and last clue inside. That led the kids to the hallway bathroom-  where my find was dressed as the "creeper"  hat and mask with all the presents.  After the mystery was solved, we opened presents and had cake and Nutter Butter (peanut shaped) scooby treats. The kids then filled their loot bags with all the thins they had collected- Scooby snack, prizes for find the clue, candy from pinata and dress up.  The kids had a great time and loved to get to dress up and solve the mystery.