What are some good ways to take a break during the academic workday?
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, ...).