Go up with Bethesda's current RPG. You'll be back before you finish it.
Seriously, though: cp library_of_congress /media/box_of_microsd_cards. Read books. Watch movies (and binge on TV seasons). Play games with your crewmates. Teach yourself something. Watch recordings from friends and family, record clips to send back. Invite tech companies to develop push versions of their services; they can't buy publicity like astronauts checking Facebook from Mars.
It's hardly isolation, and six months will go by before you know it.
I guess our biggest challenge is getting to Mars before our collective attention span has decreased to the point where we can't survive without minute-by-minute feedback from our social circle.