Wasn't sure where to put this one, but as they were free in boxes of cereals, I think Food and Drink are quite apt.
Shrinky Dinky's were a picture of a cartoon character, (I think mine was Jerry Mouse), and they were on this plastic type sheet. So you cut them out and put them in the oven, (this is vague, but I think you might have had to colour them in)? And when you got them out, they had......wait for it......shrunk! Even as a kid I remember thinking, 'and???' And I didn't have loads of stuff as a kid, so imagine how more priviledged kids must have felt! Lol!