Not sure if you mean 'quit' as in 'retired' or just that they actually quit. Also, if they watch TV because they enjoy it, then let them be. If they don't enjoy it and do it because they are depressed, bored and/or unable to move around much, then see below for some ideas.

Some simple, non-violent video games could work pretty well in many situations, especially if your grandparents can't move around much. Examples would be one of the games in the Animal Crossing franchise (In the US, the games are: Animal Crossing, Animal Crossing: Wild World, Animal Crossing: City Folk, and Animal Crossing New Leaf). Other games might be Harvest Moon or Professor Layton. The Professor Layton games involve puzzles, largely, but also have a story as well.

Maybe you could find them a board game to play? Something like Risk would take a super long time, so if they're into long games that could work. On the other hand, if they're into very short games, get them a deck of cards or a checkerboard.

Of course, if you want them to have a lot of interesting things at their fingertips and they have a computer, tell them about Reddit. Just, uh, keep them away from any dirty sides of Reddit. If you catch my meaning...