I made this construction site cake last year for my son's second birthday. It was the first "themed" cake I've ever made, and it was a huge hit. It was also VERY easy in terms of decorating. The cake itself was a two-layer 9x9 square chocolate cake, with peanut butter chocolate buttercream filling, and chocolate icing.
After searching online for cake inspiration, I went to the bulk section of our grocery store and looked for anything that might be the right colour for rocks and dirt. I ended up with several different things, including mini chocolate chips, skor bits, cookie crumbs, and some candy-coated chocolate rocks. (The rocks included several other colours , green, pink, blue, etc. but I only used the brown-toned ones). The best purchase though, ended up being leftover chocolate pieces and shavings/dust from the bottom of a bin of chunk chocolate. These were especially realistic since they were so rough and textured. (The same result could probably be obtained by smashing your own chocolate.)
I also purchased candy corn for pylons (not difficult to find, as my son's birthday is a week before Halloween), and thoroughly washed some of his smaller construction trucks. These particular trucks were part of a set of five CAT trucks. The only one not used was the steamroller.
Once the cake was iced, I started by placing the trucks and pylons. I then piled the other stuff in various places, pressed chocolate chunks into the sides of the cake, randomly threw cookie crumbs against the sides, and scattered crumbs and chocolate on the cake plate. We also put out cups of the leftover candy corn, chocolate rocks, skor bits, and chocolate chips, for anyone that wanted extra stuff on their piece.
The smaller smash cake was an upside-down cupcake, decorated in a similar manner.