Handheld gaming devices have come a long way over the years. They now have all kinds of fancy bells and whistles. In the 70s, technology was not advanced enough for realistic 3D graphics though. Take the Tomy Blip for instance: it is an electromechanical game that was introduced in 1977. It uses a light that moves in random for a Pong-like game.
You could play this game against the machine or a friend. There were 3 buttons on each side to deflect the red dot back to your opponent.