I have another idea of modifying the game (which can be used just like any of the five variants listed could be used), as follows:
Each player gets six cards, two face-down and four face-up. You can look at your own face-down cards but you are not allowed to look at opponent's face-down cards. Replace face-down with face-down, and face-up with face-up.
Each player gets six cards, two face-down and four face-up. You can look at your own face-down cards but you are not allowed to look at opponent's face-down cards. Replace face-down with face-down, and face-up with face-up.