It's weird, because since entering college (and ruling out environmental engineering, which I am so not cut out for) I've been pretty dead set on becoming an urban planner. The introductory courses I took at UCSD-- which really were more like urban sociology/economics/history than urban planning-- absolutely fascinated me. I didn't consider myself to really be a science-minded person, so I thought urban planning would be perfect.
My plan was to do anthropology for undergrad and then go to grad school for urban planning. That may well still be my plan, but in the course of preparing for my anthro degree, I took a biological anthro class this semester and it's totally caught my interest. I can't put my finger on why I never liked science before-- though I suspect it has A LOT to do with my high school teachers-- but I guess when it's applied to human history and cultures and current socio-environmental issues, I suddenly become fascinated. I totally didn't expect to come out of that course with a undiscovered passion for biology and evolutionary theory.
I'm fortunate in that biological anthropology is still applicable to urban planning, or I'd probably have to make a decision between the two. Which would be scary, because while I know I'm probably cut out for learning about urban planning, I don't know how good I'd be at doing it. Alternately, I might be interested in biological anthropology now, while I'm just reading/writing about it and no labs/chemistry/physics are involved, but were any of those things to be introduced, I don't know if I'd really grasp it.
So, I mean, I was trying to come up with advice for people who might be in similar situations, or situations where they HAVE to choose between their interests, but I honestly don't have any. Except maybe that's it okay to go for something more broad but still applicable to whatever you may want to do. That's the advice one of my urban planning professors gave to me, which is partially why I'm majoring in anthropology.