I'm 41 so quite a bit away from retirement, but I'm watching opposite ends of the spectrum when it comes to my parents vs in-laws.
My parents: My dad retired at age 55 back in 2001. Never had a concern about being bored and hasn't had an issue with it. But he and my mom stay active. My dad golfs 1x a week, goes flying 1x a week, they take 2-3 fairly good vacations per year (minimum 2 weeks), they go out to eat with friends a few times a month, they have grandkids in Houston they go visit, and I have 3 kids they babysit a couple times a month. They also try to go out to Town Lake a couple times a week to go for a walk than out to eat somewhere. Up until 2 years ago, they had a boat on Lake Austin, and they'd just go out cruisin' during slow hours during the week. And my dad reads a ton, and always has, and that hasn't slowed down at all. They've had a great retirement and he hasn't thought twice about it or even halfway considered taking a part-time job.
My in-laws: My FIL is a 71 year-old OB-GYN and will die at the hospital. He took an Administration job at his hospital a few years ago (in addition to running his own practice) and the hours have just about killed him. He "resigned" from that position a few months ago to be effective June 1, but unbeknownst to the family until 2 weeks ago, he rescinded that resignation. So he's going to keep working 14 hour days for the next 2 years until his contract expires, and then he'll be back to just his one job running his practice. The family is completely bitter over the whole thing since he was absent for every major event when his kids were growing up, and now history is repeating itself with his 7 grandkids (#8 on the way). The problem is when I step back and look at him, he doesn't have any real hobbies. He likes fixing things, and I'm sure he could stay busy for a few months getting things done around the house, but eventually the work dries up. My guess is he's scared to death of not working. Being a doctor is about the only thing he truly loves doing, his patients love him, he can't stand spending time with his wife--so he figures, "why retire".