ARTIST TRADING CARDS!  These 2.5″ x 3.5″ cards are the same size as all baseball, football, Pokemon etc.  trading cards.  Students create their own small artworks, sign and date the back, and trade with each other.  We use these in 3rd, 4th and 5th grades.  They can create and trade them whenever they have free time in the art room.  en.wikipedia.org/wiki/Artist_trading_cards