My daughter keeps telling me I should retire now ... and then babysit her kids for free.
Not a totally unappealing idea, but I'll probably do something more along the lines of what my dad did when he retired.
He started working as a volunteer with my mom for a prison reform group in Louisiana. He probably worked harder as a "volunteer" than he ever did at his job. My mom and dad eventually became pretty significant members of that group and used to travel quite a bit to conferences around the country, meet with politicians, and so on. Plus, equally important, it was something he and my mom accomplished together.
He hardly ever talked about work the entire time I was growing up. When I visit my mom and dad now, all my dad can do is show me scrap books of all the awards my parents won, all the conferences they attended, and pictures of them with all the various politicians they dealt with.
I'd say his life got a lot fuller after he retired, even if it meant less leisure time.