Legos are one of the few things that appeals to all the kids in our house given their diverse ages. Sure, Baby B's are Duplos while J's are Mindstorm robots, but they are still Legos and the basic concepts are the same. (Miss M's are pink and purple, of course.)
The other day, I set up an activity I had seen over at Quirky Momma by drawning a rainbow on a large sheet of paper. I had J haul down the large bin and the kids played together to create a rainbow of Legos.