A water to air intercooler;
Replace your muffler with one of these, run a water line in/out, and power a small pump to make electricity. (a la BMW, but much simpler).
Theory;
The drop on these intercoolers is VERY low, about .1 psi. The drop on a normal car muffler is of excess of 2 psi. So not only will you get a heat exchanger, but also a free-er flowing muffler.
Since they are made to effectively convert temps from water-air, it will effectively convert temps from air-water.
I think this is the best thing to use as a energy recovery system and easy to implement... what do you think?