I usually read something that is on my reading list, either a physical book or a pdf on my palm.
Sometimes I write down acknowledgements and graditudes into my journal, sometimes I jot down notes on projects. On some days I start my GTD weekly review in the bus.

I know that many of those things will not work for a person who gets motion sick in the bus, or who drives or walks to work.

Walking I consider in itself to be productive as it promotes health but it can be combined with a useful audiobook or podcast, too.