I try to minimize the rebound time. One thing that helps me with that is to reread Paul's essays. (Is it bad to admit that? It must look like kissing up. Whatever, I don't care.) They're short enough that I can sit down with one and tell myself "Okay, when you're done, get back to what you need to do." I feel sort of.. revitalized afterwards.
Head down to the Ferry Building; that's where the summer sun shines in SF. Indulge in a glass of red wine and watch people.
Change of context: book, film, game, chat... Preferably something that keep the brain ticking over, but allows the 'work' parts to take a rest. Oh, and virtually any kind of exercise.