I haven't heard back from some schools for interview offers. Should i send them an email saying I will be in the area (i will be for winter break), or should i write them a letter saying that?
I would just use email. It's faster and more convient for both sides. Also, unless ur schools really far away from ur home, i.e. maybe like east to west coast, i'm not sure how much of an effect that will have towards u getting an interview invite. you might be better off sending a letter of interest. as far as i've heard, there are very few schools that are receptive to "in the area" emails, mainly NY schools like cornell and columbia. good luck though!
Also, from what I understand, an "in the area" letter won't get you any interviews that you wouldn't have gotten otherwise. You might get your invite a bit faster, if anything.