We had a Scooby Doo themed 4 year old party. I built the cab of the Mystery Machine Birthday Cake out of cardboard and iced it with Royal Icing to harden it up and make it sturdy. The back of the van was made with 2 9X12 cakes cut so that a third layer could be made with the remainder.
The trim was made with gum paste. The decorations and lettering was made with Royal icing onto parchment the night before and then stuck on. The wheels are “pinwheel” cookies with the hubcaps iced on. The rest of the icing was a regular buttercream.
The whole thing was elevated using Legos. I picked up the set of Scooby Doo characters at Toys-r-us for about $16.