Build a blinking LED wall with Trilife
Build a bunch of little circuit boards using these design files, then connect them together to see patterns emerge. Included in the download are a pdf with detailed build instructions, the layout and schematic (in Eagle 6.0) for a single board and for an array of boards, a bill of materials, and PIC firmware so you can program the boards. It's designed for a large group to learn how to solder.