Noughts and Crosses
This is the classic pen and paper game, and with good reason. The rules are simple and easy, and games are usually over quickly. However, it’s also a game that can get boring quickly. Between two reasonably experienced players you are always, always going to get a stale mate.
However with a bit of imagination you can create all kinds of variations. For instance, change the grid from a 3 by 3 grid to a 7 by 6 grid, and introduce a rule that you can only place your O or X at the bottom of the grid. Suddenly Noughts and Crosses are now Connect Four!