Materials: Deck of cards
each suit represents a different push up. I do diamond, regular, yoga, and fly push ups.
Go through the deck and do the face value of those cards. Jack are 15, Queens 20, and Kings 25. By the end of the deck, you get a decent workout for being in transit.
I also carry a couple of cardio DVDs with me wherever I go in case its too cold to get my cardio on. Some of them aren't too bad, although it's good if you have a HRM and can guarantee your heart rate is sufficiently high.