Lego Art -- Use random bits of legos and have kids stamp with a black ink pad. Use a silicon baking sheet under the paper to give the needed "squidge" for the solid plastic Lego to make good contact for a decent looking stamp. once dry, have the kids color in their creations with crayons or pencils.
I will need this in the future for Paul & Kade! ;) 37 Genius LEGO Organization Ideas - Kids Activities Blog