Laura of The Scoop made this graphic mobile that we love. Modern and high-contrast to engage young eyes, it's also easy to make. She used a metal photo hanger from Urban Outfitters, but helpfully points out that you can purchase a similar kit at Giggle ($15). She designed a few of the graphics herself, but also used Zapf Dingbats and Webdings. We're sure you can find some simple clip art to use or even draw something free-hand.
Read Laura's post for more information.