I would like to address this question, because I was so busy I had to back off and stop doing a few things so now I am actually a little bored at times but I know what to do to fill up my schedule again, and for me it has mostly been volunteer activities.
The thing about volunteer work is most is grunt work & not mind challenging so there is a limit on how much time you want to spend doing this.
<== This is very true and there are some things I do for exactly that reason. Once per week I deliver meals with a local "meals on wheels" program. Pretty mindless, but I get to briefly see some very nice people who are always glad to see me and appreciate what I am doing for them. I also volunteer as a Guardian ad Litem, which not only helps children who need help, but fills my need for "professional type work." I prepare written reports that go to the judge and put on a coat and tie to attend court hearings. You don't have to be a lawyer. If you are at all interested in child welfare I would encourage you to check it out. They are also called "CASA" (Court Appointed Special Advocate) in some states, but almost every state has a program. You are not a mentor and you are not the child's baby sitter. You are the child's advocate and you have to independently investigate the child's medical care (talking to doctors, dentists), education (visiting schools and talking to teachers), and caretakers. You have to develop credibility with the judge to the point that he (or she) values your input and takes your recommendations seriously.
My work as a Guardian Ad Litem got me interested in foster care, and through a different charity my wife and I hosted a child who was in America for medical treatment. This turned into a 6 month stay, and the child's needs were great, so for that 6 months it was like a fulltime (24/7) job.
So I have currently eased off a little on my volunteer commitments and trying to get back into golf a little bit. I have an application on my desk to volunteer at the local hospital, something I have always wanted to do. The local police department also put out a request for volunteers and I am considering doing that just to get an inside look at the police department.
I exercise more regularly than I possibly could if I was working fulltime. I play guitar and piano and read more than I could if I was working fulltime. Haven't felt the need or desire to take classes, but I could see doing that too. There is plenty to do!