Regarding the "pin" the tail on the donkey -- another idea is to "velcro" the tail on the donkey

it's so much easier for children of this age.
For my sons birthday he did Bob the Builder and we used the "dirt cake" in the back of a toy dump truck in addition to the Bob cake we had. It turned out to be a great hit! You can also do the recipe with vanilla pudding and nilla wafers to make a "sand cake"
Here's a few more "construction" themed ideas;
Boulder Toss
Divide your little workers into two equal teams in separate lines. Give the leader of each line an inflated balloon. At the sound of go they must try to pass the balloon in the air to the next person in line without it ever touching the ground. When it reaches the end person have them pass it back to the front. The first team to get it to the front of the line wins.
If you're having an afternoon party ... why not pack up some lunch boxes or bags for those mighty workers. Fill it with a juice, a cookie, sandwich, some treats, etc (really cute if you can find some inexpensive lunch boxes or colorful paper bags!
Have them "construct" their own rootbeer or coke floats!