I have a degree in mechanical engineering and am very into alternative energy. My senior design project was a through-the-wall solar oven, and after it was finished we donated it to a grad student at our school who was working on designing and getting inexpensive solar ovens into homes in the third world. I am also a big fan of astronomy, math and physics. Biology makes me laugh. Chemistry makes me cry.
Geology ( I'm studying to become a palaeontologist) , Biology, but most fields of science pick my interest to some extent ( to a broader extent that's what it means to be a geologist, you study the earth, so everything that happens on it will interest you in one way or another)
So far I've been rocking environmental science, biology, and chemistry. Quantum mechanics sounds interesting, but I haven't really done anything with that. Psychology is fun to pretend to know things about, but I wouldn't trust myself with actually applying it. I also like genetics. The final thought on all of this is that I think I would like to major in microbiology.