When my son turned four we gave him a construction party. The invitations were shaped in a mighty machine and told the guests to come prepared to work.  Once the guests arrived we had construction tape across the front lawn leading them to the back yard where there was a marked off area with orange cones and caution tape.  Inside was a ton of mighty machines and dirt, they had a blast, even the girls.  We decorated the tables with old blueprints and served nuts and bolts in plastic constructions hats from the dollar store.  The cake was layered in the back of a tonka dump truck to look like a pile of dirt, this wasn't enough cake for all our guests so we added a sheet cake behind and made it look like a construction zone behind the truck with yellow lettering and cones and candles shaped in different machines.  There was a shovel sticking out of the dump truck for my son to scoop out peices of cake to his guests in tiny construction hats.  We played a lot of fun games.   We had a building race, I saved boxes, like cereal boxes, formula boxes etc till I had enough for two teams.  We divided the group and had them race 1by 1 one to build a tall building with the boxes the first team to win got candy. WE then had an obstacle course the kids had to race using a dumptruck wearing a construction hat and goggles they raced over a ramp through the dirt and through a tunnel made from cardboard the first team through won. We also had a scavenger hunt for tools, again we used old blue prints for the map. I found small tonka trucks on sale and gave these out as party favors. The kids had a blast and it was a lot of fun for us too.