Cornstarch paint was a big hit with my 5 and 7 yr old charges last summer. They enjoyed helping me set it up (and it only took about 5-10 minutes). As for your younger charge, my 2 and 4 yr old nieces really liked it too, though they were less able to help with set up.

Here's a how-to link: http://happyhooligans.ca/sidewalk-paint

Sidewalk chalk, bubbles, paint the sidewalk with water,
home made play-doh,

Over the summer I go from having just a toddler to having two school age charges as well. The are 5 and 6. I would like suggestions for activities that don't require any prep and few or no materials. Because I will not have nap time to set up and I will be more tired then usual I would like to be able to entertain them on the spur of the moment. The older kids are good about playing with each other so mostly I need suggestions for two year old activities.