Build a Maze: the two younger boys worked on this:
Next, I had seen a "Cup of Legos" Challenge: fill a measuring cup with random Legos, set a time limit and go!
We had all sorts of creative pieces build: a dragon upon which a Harry Potter character rode, several helicopter-like vehicles, a couple villian's lairs with various contraptions, a Parkour play gym.
|Parkour gymnasium flanked by a dragon (L) and jet propelled helicopter (r)|
The boys really loved this challenge, and it kept them busy for a good hour while I cooked up some dinner.