Pad of post-its, pen
Keep a pad of post-its with you when you go out. When you need to pass a little time, take out ten, and you or your child can write Xs on half and Os on half. You can then have an impromptu game of tic-tac-toe on any table or wall that is nearby. With an older child, you could also write a series of letters (e.g. A, T, C, M, R, H) and see how many words your child can make from them.