Chris, the MOST important thing about being a good passenger is Jelly Beans. They are essential on long trips!
Secondary to that I spend most of my time looking for good places to land (just in case), watching the instruments on my side of the dash, following where we are on the map (even though we have a GPS and moving map system, it's challenging to do it with the map only), watching out for other planes, frequency changes and general amusement duties. Singing, jokes, story telling, picking out strange things to see on the ground etc etc. When we hit bad air I generally try to not flail about or scream uncontrollably!
What I like best about flying (aside from all the great people we meet) is the planning. Maps, route planning, frequency changes, ERSA, fuel consumption, flight plans... it's a logistic dream!!