Great ideas and i especially like the audiobook idea! For me it depends whether I'm going on the trip or returning home. Going, I tend to organize my notes and read about some of the sights I'm planning on visiting. That used to be in guidebooks but now it's a combination of guidebooks and material I've printed off. Coming home, I try to relax and unwind. Maybe watch a movie or read a fiction book.
My next trip I want to use that time to learn some French.