At nano sizes, evaporative cooling is probably your best bet. Top off water and fans.
You can use peltier junctions (and some off the shelf small chillers do this), but the efficiency is kinda crap, and the actual amount of energy they can move is also really low.
Above this you're looking at small refrigeration compressors and using a titanium refrigerant coil, which is pretty large for nano size. If you're into refrigeration systems and have the tools, its fairly easy to do, but does take an investment.