Playdoh with a plastic knife, rolling pin, cookie cutters, small plastic toys, etc.
And these very helpful links:
http://www.geocities.com/parentsworkshop/act.html (this site is down now, but thru the magic of the wayback machine at internetarchive.org, you can see it again here.
http://www.redshift.com/~bonajo/preschool_activities.htm *tons* of stuff do here- you’ll never want it all, but will find plenty to keep them busy!!!
When my first two were small, I wrote down lots of ideas on index cards for those frazzled days when I needed something quick but couldn’t think for the chaos (our eldest was rather active herself!). Until just a few very short years ago I kept an index card of ideas in my wallet still for when we were traveling or out on errands and I needed some distraction for them.