Bunko, small paperback
What do you do on the train if you have to get on a train for commuting for a hour, as is very common in Tokyo? I would read some books, listening some music with iPod.
Many piles of A6 size, 105mm x 148mm, paperbacks, called Bunko in Japanese, are sold at bookstores in Japan and very popular among people commuting on trains. Such books will cost us USD 3 at least, USD 7 at most and more reasonable than the hardcover ones.
I believe that books are one of the best investment and that good food and reading make us health physically and mentally.