What are some fun, sensible things to do on a long flight?
(fun thing to do for a sensible grown up...
i.e - not childish or immature)

1)
Burn those vacation and family photos to a disk, the ones you have not seen in awhile, then view them using your laptop.

2)
Think of two people you have not spoken or written in some time and write a very short message on a postcard, you can hand it to the head flight attendant to mail for you. I usually like to write some old college buddies.

3)
Check all bagage, except things that can fit under your seat. I say this as, I want you to wait until the end of the boarding call to get on the plane. This will keep you from sitting in your seat an extra 30 minutes when you can just get on at the last minute prior to departure. The bad thing with this is that you might not have space left in the overhead compartments, so this is why I suggest checking all baggage.

4)
Get up and walk about the plane. You are not required to sit the entire flight, and it can even be unhealthy to do so.

5)
Watch the inflight movie, and request an isle seat. You will be more rested once you arrive at your destination.