Lately, when the kids have been bored on public transportation, we've played brain workout. I say a word backwards and they have to figure out which word it is without writing it down, just listening to it and twisting it around in their heads.
The longer the word, the bigger their smiles when they nail it.
Ps. Unexpected bonus - makes your own brain work too, as you're turning the words backwards for kids. Or other bored commuters of choice.