I always tell folks the Automation (autopilot) in an aircraft is like a student pilot. It will do exactly what you ask it to do. It does not use judgment, reasoning, or pilotage to make changes. Always monitor what is going on. On long flights, maybe 4 hours, you will find that there will be a decrease in action. Talk with your copilot, read up on the aircraft performance stats, procedures, destination weather, enroute weather, make a few pilot reports, double check your planned flight against actual fuel burn and routing, enjoy the scenery and hand fly a bit.
If you are time building then I assume you are doing it to build your experience. DONT waste your time and money doing things that dont build experience. If you want to zone out for 2 1/2 hours of a 3 hour flight then just buy an airline ticket and rent a 152 at the destination for a takeoff and landing. Yo owe it to yourself to get the most out of this experience.
Flying as a crew in a light twin or single can be great fun and very rewarding with both experience and personal company. If you have aspirations of professional flying (GA flying is professional too) then dont Ipod and Zone out. I recommend the above, you will reap the rewards later, trust me.
Good Luck