This is the easiest thing to do. You can use unwanted CDs simply as coffee mats, silver side up, or picture side up, if you prefer. If you want to do something a bit more arty try this:
Once the glue has dried you can paint the surface of the CD. Use enamel paints as these dry as a hard, heat resistant surface. Lightly sandpaper the surface of the CD before painting it. The best technique is to lay down the enamel in layers. For example to make this smiley face picture (on the right) you paint the whole disk yellow. Once the yellow layer has dried, paint the red smiley face. The nose is made by painting inside the hole at the centre of the CD. Finally, to make the whole thing really hardy, give it a coat of clear, gloss polyurethane varnish. You can, of course, paint far more interesting pictures than this.A whirligig is traditionally a circular, brightly painted piece of wood with a central hole.

Through this hole runs a wooden "axle" which has small circular end stops at each end. A loop of string runs from one end stop to the other. You hang it up from a loop at the midpoint of the string and then wind up the whirligig until the central wooden wheel fits snugly inside the string. Then give it a gentle unwinding turn and release it: the wooden wheel unwinds and rewinds, up and down the string, and the painted design forms patterns as the wheel spins. In this case you simply substitute CDs for the wooden wheel.
Drill the dowel with a central hole just wide enough for a 4mm dowel to fit inside. You may have to drill this by hand; if so try to make sure the hole is both central and straight through keeping parallel with the sides of the dowel. (You will probably need an adult to help with this).diagram of dowel.That's it. Hang up the whirigig, wind it up and watch it spin!