You know your child best... does he sleep well? Will he sleep easily in a new place, with lots of commotion? Does he get agitated when overtired?
I took a trip with my son when he was about 15 or 16 months old. My son was never a great sleeper, and hoping that I could get him to sleep on a plane for naptime was, well, not wise for us. We actually planned on him to be well-rested instead. I packed NEW toys and some interesting snacks (fig newtons, yogurt covered raisins, chocolate graham crackers... new and special treats). I remember having a few new books that I thought would appeal to him. Check out the dollar store or cheap toy section of Wal-Mart for small, new things.
Wouldn't you know, our flight was diverted (something about having to fuel up because of the wait to land at our destination airport). It turned out to be about 5 and a half hours of sitting on a plane. My son was social, standing on my lap looking around and smiling the whole time. My neighbor on the flight let my son look at and try on his hat. This kept him occupied off and on for about an hour! I was terribly exhausted from trying to keep him from mischief, but he was a champ.