Besides mentioned by others walks and sleep I found meditation to be really helpful; you often can get the benefits, ideas-generation-wise, of a full-night sleep doing solid 30-minute session. There are other benefits of meditating too :)

Agreed -- and IME even a 10m zazen session is helpful. Also, a 20m nap can be surprisingly restorative. Physical movement (even a couple min of stretching) goes a long way, as does changing the environment (switching rooms, and/or writing design notes by hand instead of typing).