peltier element

use an h-bridge with a temperature sensor attached for monitoring

an h-bridge will allow you to turn it either hot or cold (reverse polarity) using 2 pins

Its the same type of component that allows forward and reverse on a motor only no protection diodes are needed since its not an inductive load.

Here is an example for a motor using only 2 pins (I would use a couple more transistors to make sure both couldnt be in the on state at the same time as that would likely cause smoke

http://www.bobblick.com/techref/projects/hbridge/hbridge.html

or a possibly more effecient design using MOSFETS here

http://www.cadvision.com/blanchas/hexfet/np-s.pdf