We celebrated Max's third birthday a few weeks ago - complete with a construction cake. :)
Easiest cake ever. Seriously. If you have a kid (or grown up) that likes dirt and/or construction, you must do this.
Make a chocolate cake. Flip it out onto a pan, then break it, crumble it, smoosh it, pile it up. Top with some chocolate frosting in creative ways, add a few piles of rocks (chocolate chips), accessorize with a bulldozer and candles, and you're done! It was a huge hit. :)
I can see this working really well as a groundbreaking cake, as a gardening cake, as a worms cake... so many possibilities. And no pressure to have a perfect cake!