This would also be cute in a lego museum. Make a quick and easy Lego zip line to test out slopes, angles, gravity and tension plus engineering skills. Lego zip lines are fun!
Lego Balloon Car Building Activity Lego Race Cars Kit - blow up the balloon and the car takes off
Build a LEGO Design Drawing Machine - Similar to a Spirograph, this draws awesome patterns! - possibly use littlebits instead of lego power kit?
Count Down to Christmas with a DIY Lego Advent Calendar - Frugal Fun For Boys
Fun for GIRLS & boys: LEGO Building Challenge: Machines + Paper. How to build a working LEGO paper crimper and a machine that draws circles.
LEGO gingerbread house by Chris McVeigh