Sparkfun is coming out with a new breadboard that includes a Arduino in a LEGO compatible base. All the ATmega328P and ATmega16U2’s I/O pins are broken out and available. The breadboard has shield compatible pinouts (yes, the weird offset is there). A bit more than you would pay for a Arduino UNO, but the convenience of having a breadboard and complete access to all the pins makes up for it. This is a perfect teaching tool!