If I'm anything like my parents, I don't think that's something I have to worry about. My dad is constantly buying new textbooks and trying to teach himself different types of physics. Either that, or he's designing new things to be 3D printed.

He's not retired yet but I suspect that when he is he'll find a way to keep himself entertained with stuff that isn't terrible game shows.

I've heard that you can expect your retirement to look a lot like your average weekend. If your weekends naturally fill with your hobbies and interests, I don't think you have much to worry about.