We just got back from a trip with our three little ones (all under three) and had a very easy time with it (caveat - they all had their own seats but I had someone on my lap for most of the trip anyway). There are a few things that made it easy:
1. Fun games for when you are sitting down -
This little piggy with toes, patty cake, hiding small items in one hand and making them pick which one, some books with colors so they can find things with those colors, if you have a computer out they can type, of course the DVD player if they are willing/interested in that.
2. Singing Songs but changing them or creating new verses - for instance, singing "Oh Susanna" with the banjo on different body parts. Or singing your favorite songs making things up or singing them incorrectly...they will want to correct you over and over again so it lasts a long time.
3. Keeping food and other items at the ready for when hands just need to be occupied.
The hardest part with my son (who is just a little bit older than yours) was that he kept kicking the seat in front of me. So at some point I just had to turn him around facing me and have him play looking at me.
We weren't successful with napping on the lap - I hope you have a better time with it.
Good luck and have a great trip!