For my son's 4th birthday, he asked for a Monster Truck cake. I ended up getting the Wilton 3-D car mold and cutting to be shaped more like a truck. It was easier than I thought, as all I did was make the sides more square and flatten out the back to make it look like a truck bed.
The mold took 1.5 cake mixes, so with the left over cake mix, I made a pan of cupcakes. I put some of the cup cakes under the truck to support it and give it more lift. I crumpled some to make it look like the dirt on the 'road'. Then I made 2 13x9 pan cakes and put them side by side for the truck to sit on. I had to do this as the Wilton mold was bigger than I thought. I filled in between the cakes with other cake pieces.
This was much easier than I thought since you want the cake road to be imperfect to mimic being off-road.
To decorate the truck (my son insisted on a blue truck), I used the gel food coloring (mistake #1 - I used sky blue instead of navy so I couldn't get it dark enough). I tried to use a icing tip, but the icing melted too easily (mistake #2- don't buy the whipped icing if you need it firmer). I used store bought black icing to outline the truck. Mini marshmallows for the flood lights on top with chocolate icing. The grill on front and back are chocolate wafers halved so they weren't so thick (very easy and they stuck by themselves).
The windows and headlights/tail lights are white store-bought fondant. It came in a brick (like cheese) and I used a cheese slicer to cut thin pieces for the windows (no rolling of the fondant required). I used a round toy from my son to punch out the headlights and tail lights from the fondant.
The wheels are crueler donuts that I dipped in chocolate icing. I was going to use melted chocolate but ran out of time. I melted the icing in the microwave to almost liquify and just dabbed the donuts in it. Easy but messy. Mistake #3 - I used donuts baked fresh as they were VERY soft and almost broke apart. I recommend using donuts that are a day old so they stand up better.
We had about 20 people and a ton of left over cake. I got lots of compliments and the kids just loved it.