Retirement is wonderful, you should all try it ;-) Remember I did work full time from 25 to 63 (that was after grad school, which is a full time job in itself), plus have a family, so I didn't retire as young as most of you are aiming for.
Apart from the frivolous life I am leading (theater, curling, various clubs and hobbies I never had enough time for before) I am doing more volunteer work in my community. My dog and I tried out as a Therapy dog team, and after passing a tough test (50% pass rate for our group) and having three supervised visits we are now on our own. We will be doing visits once a week to a local senior's residence. This is a rural area so many of the residents had dogs, and really like to see her. Down the road I would like her to participate in children's reading programs - many children will read out loud to a dog when they won't do it in class or at home, the dog is comforting and non-judgmental. My dog loves loves loves children (breed characteristic) and she is very cuddly, so I think this would be a good activity for her. We have to have a minimum of a year as a general therapy dog before she can be tested for that, though.
Philosophical musing - We discuss quality of life and goals here a lot - as people plan their retirements, and really even in their daily lives before then, I think contributing to your community in some way adds meaning to life. I have done a lot of volunteering, was involved in community activities, was a Scouter, was active in associations connected to work (but not things I had to do because of work) - I can look back and see that things happened and (hopefully) were better because of my contributions, and those of other volunteers.
Also there has been a lot of discussion on the characteristics of a good spouse in a Mustachian way. There is more to it than attitudes to money - my almost-ex husband thought volunteering was for suckers, our values did not exactly align. Telling your spouse she/he is a sucker does not lead to a happy marriage. So finding someone who shares most of your values, not just those about money, is important.