My DS is 4, but I also teach Kindergarten (5-6 year olds)...here are my suggestions...
The Dollar Spot at Target has a plastic pretend "town" map, plastic buildings, and matchbox size cars. I bought the "map", 4 of the buildings, 4 cars, and even a white bag meant for toiletries to hold it all. DS had a great time spreading it all out and playing with it.
When we went to Disney in January, I found a set of Matchbox cars that were safari themed since we stayed at AKL. That occupied him for many, many hours!
I bought him a travel Etch-A-Sketch, and a travel Magna-Doodle, both of which worked very well in the car. (I read here on the DIS boards of a family that actually takes the Magna-Doodle with them to the water parks...they can leave each other notes about where they are. They also make magnetic ones, so you could leave notes on it and stick it to your door.)
At school, the girls and boys all love the Travel Lite-Brites, Uno game, and the pattern blocks. If you use the Travel Lite-Brites on the plane, be sure to buy a Sterilite shoe box (~$0.99) for them to set in their lap so that the pegs don't fall on the floor. (Although my son would find a way to upend the whole box, I'm sure!

)
HTH!!
~Kimber