If you live anywhere near Cambridge, MA, the Edgerton Center at MIT offers several free LEGO kit science classes. I arranged for our homeschool support group to participate in the Car Rally class because that one allows for the broadest range of ages. Two other moms and I brought in 14 kids; Dh personally escorted us directly to the Center. Here they are heading towards the Infinite Corridor.
Kids were broken up into groups of two and given Lego kits with which they built cars. In order to earn points, they first had to put the cars on a tilt board; the higher the board could be tilted without the car falling off, the more points you got.
Next the kids brought the cars to the track in the hallway. The further the car went, the more points they got. They needed to try the cars both forward and backward--often backward worked much better! Then you went back in to fix your design and try again.
The class ran for 3 hours, including a 30 minute lunch break. Since we all left our lunches in the vehicles we went for 2.5 hours straight through. Only the youngest two showed any signs of wear; the rest we had to almost wrestle parts from their hands!
Amy, the class facilitator, is at the tilt board. Below is the track in the hallway. I highly recommend the classes if you can get 6 or more kids of the right age for them. MIT offers parking passes for $12 per vehicle if public transportation won't work for your group. We bought 3 passes and that was still less than $1 per child. It was well worth the expense.