Book club, definitely.
This is, in my opinion, the option that brings you into contact with other people, more likely women, on an equal footing where you will most likely exchange info about each other and can decide which of the individuals you might like to meet outside of the club for other social activities. You'll get to read some fabulous books and exchange opinions about them, hearing the ideas of others which can help broaden horizons in a way that challenges your brain, and helps keep the synapses firing, in a way which is radically different from your legal work.
Volunteering, especially offering legal advice, may just seem an extension of your day to day employment. Volunteering, I have found, does not actually make you feel that good about yourself. You end up doing a lot of grunt work. Yes, some people will thank you for it, but I have found that it does not change me, or my opinions about the world all that much, it has not made me a better person, I am still me, and I find I tend to get irritated by some of the infrastructure that requires things to be done a certain way, not letting you use your initiative and just get on with it. Maybe it's just me. But I really do think that the "do something for others and you will find untold depths of humanity within yourself" idea is somewhat overrated. Also, the people you meet will be those for whom and to whom you are rendering a service, and it is unlikely that you will make many friends this way because you will not be on a equal footing with these people for they are inserted in society at a different level of power/disempowerment than you. Maybe you could make friends with the other volunteers, maybe not. I help out at my kids' schools all the time and none of the women I work with and alongside are at all interested in hanging out for coffee afterwards or at another time. No, they have lives that are overpacked and overstressed and they dash off home to see to the next thing on their "to do" list. And I think this is the problem with the exercise class also: you dash there, the focus is on the exercise itself and the instructor, not on getting to know your fellow exercisees, then people dash home again afterwards. I would put this option second in place, after the book club.
At the book club, you will find that only part of the discussion centres on the text in question, the rest of the evening bis pure social gathering, having a drink, some nibbles, and it is more conducive to meeting women who think as you do.
It is a shame I don't live near you, I'd love to start up a book club with women just like me and of this age-ish. That's what I would do. Maybe I'll start up a club of my own for women in their forties in Ann Arbor, MI?
Good luck Nancy and nice chatting with you. I certainly understand the "need for other avenues of self-expresssion and meeting new people" desire. I feel it too.
Clare