When my son was turning 4, he asked for a "construction" party. This cake is the result. We also had yellow "hard hats" for the kids that we got at the local party store.
The cake is a standard Devil's Food, baked in 2 8X8 layer pans. I did not cut the tops of the cakes as you would in a layer cake, as the rounded shape adds topography to the scene.
I used simple cream cheese icing, colored a dark tan. (Brown tones may be achieved with 3 drops yellow, 2 drops red, and 1 drop blue.) The "road" then is made in two parts. The "paved" road is ground up Oreos (take out the cream before putting in the blender!) lined with white icing. I held a butter knife on the cake to press the Oreos against so that I got a sharp edge on the roadway.
TIP: If you don't have a bag to do the decorating, use a plastic baggie with a carefully clipped corner. It's not as precise, but it's quick and cheap.
The remainder of the road is "being built." I used a combination of ground Nilla Wafers and ground graham crackers to get the right coloring for the roadbed.
The stream through the cake is blue Jello using the "Jello Jiggler" recipe: less water means firmer Jello. This was cut into a strip and laid on a foundation of "rocks". The rocks are deformed M&M's. You can make your own but I bought mine (of several sizes, as you can see) at a local candy store. If the top surface of the Jello ends up too smooth, it won't look good, so I poked it a bit with a spoon once it was in place to create more of a rippling effect.
The bridge "beams" are Pirouette cookies. The advantage of a construction site is that the bridge need not be finished/paved. The one Pirouette in front was there to prevent the Jello from sloughing out... although the jiggler held up really well.
The trucks were an inexpensive set I found at the store. Note they were posed, with a dump truck dumping "sand" and another with rocks being hauled away. The trucks made for an extra birthday gift. There was also a candy-LEGO structure, but this was really unnecessary.
The cake was a huge hit and my son still talks about the Jello River.