• Mixing plates for epoxy
• Target practice
• Prop up a crooked lamp or piece of furniture
• Use packing tape to strip off the reflective layer and admire the transparent disc
• Corollary to above: cut out rectangular pieces as small windows for project enclosures
• Corollary: replacement "lens" for flashlight
• Crappy disposable frisbees
• Use as reflectors to take revenge on neighbors for light pollution

I used to cut them up to use them as kaleidescope mirrors. (The reflected colours make them almost better than normal mirrors)

Edit: I just remembered another one! tie multiple along a string and hang it up as a bird scarer. (Bonus if you make some other place for the birds to be happy (as repayment))

Coasters.