Suggestions for those kids that whip through their art projects: Have them
do the same project again (if this is feasible) but this time from a
different viewpoint. Perhaps as a visitor from another country or even
planet. Maybe with their left hand. Also, for older kids, have them follow
a critique of their swiftly completed project and see if they are really
"done" or can some improvements be made. Or they can help other kids.
I have a section of my room for the swift kids, it contains freedraw paper
(recycled from the copy room), art books, and an art history memory game.
Usually the kids will take a "how to draw your pet" or similar book and
work on that while everyone else is still on their project. Littler kids
sometimes like making the project a second time, too.
good luck and if all else fails, have them clean something, like tools or
the sink. Maybe its mean, but its sure is helpful