>The only problem with my theory is that this effect (ie. not wasting time) lasts a couple of days after returning from a trip. This should not happen because you are in the same context as when you were liberally wasting time.

When you return back home, home is somewhat a different experience from the day to day drudgery. I think it's just that our brains hate boring routines and modern capitalism has made your life extremely Specialized and routine.