What are some good ways to take a break during the academic workday? Solving research problems is hard. After a while I feel mentally spent. I want to be as productive as possible but I don't know of many good ways to "recharge my batteries" after I have been thinking hard.

I have tried taking walks but in the winter it is snowy and cold outside. I have tried socializing with my colleagues but I find it hard to focus when I get back to work. I have tried switching to easier tasks (like email) but that doesn't seem to recharge my energy levels much.

What are some good ways to take a break so that I can come back to my work refreshed, energetic and enthusiastic?
Getting back into "The Zone" always takes some time. Relax, while nervous it just won't happen. Walking seems to do the trick for you, perhaps warm clothes help when taking a stroll. Socializing doesn't for you (and I doubt it would work for me). Perhaps read a chapter of a book (not technical, a novel or so). Or try meditating, or yoga. Or go to the gym for a half hour or so.

And don't get all worked up if it just doesn't click one day. In my case, when nothing works, I just give up and do "chores", like chasing after references to complete bibliography entries, check it there are new versions of documents/software packages out, edit my notes. Or do my course-related work (invent homework/exams, ...).