One way to mill boards from a tree is with a chainsaw mill which pulls a chain saw through a large piece of wood.
The chainsaw is typically moved by hand and the user slows down and speeds up depending on how hard it is to cut the wood, which precludes attaching a constant speed drive to move the saw.
Recording the sound of the saw and analyzing its frequency content might be able to be done acoustically, and the chainsaw motion controlled accordingly – perhaps a good E90.
Design some kind of sensing network using the TI sensor tag – available in bluetooth and wifi.
CC2650STK Texas Instruments | Development Boards, Kits, Programmers | DigiKey
Title says it all. USB, Ethernet, Video in and out, more processing power than beaglebone. Requires an external JTAG controller.
A robotics kit from CMU developed with children in mind. It has a visual drag-and-drop programming paradigm.
APC has developed a 49$ Android computer. This might be an interesting platform for an embedded system for somebody who wants to learn app development on an Android system. It should have very powerful low level control of hardware and a very nice user interface.
Do something with a small FPGA package (in a USB dongle).
Altera BeMicro Embedded Control Made Easy.