In a small Ziplock bag: cup half and half (milk works too), 1 Tbsp sugar, 1 tsp vanilla. Insert that bag into a gallon Ziplock filled with ice and salt. Shake the bag for five minutes. Then remove the smaller bag which should have turned into ice cream!
Crayon hot rocks: If you have not done this yet, you need to. Simply warm up the rocks in the oven (I did 220 for 10 minutes), then put them on a stack of newspapers (or something else you don't care if it gets crayon in it). Just remind them it is hot.
Ice painting is a fun way to cool off during the hot summer months. This website even has a substitution for children under the age of two who will want to put the ice paint in their mouths: kool-aid ice.
How To Make Flubber, Glurch and Other Homemade Art Supplies at Home