All great ideas in here for when you get tired of the beach and beer. Volunteering, be it at HH, a school, the museum, etc; are all great options. Club sports or triathlons/marathons/etc are great to get a social group beyond A-pool. Plenty of ways to learn new skills or perfect existing ones.
If I had that much spare time again I'd probably do something like this:
-find a volunteer activity to give back. plenty of youth activities can always use help
-find an activity/sport to get me outside with a group of folks. for me that might be golf, shooting, fishing, archery, hiking, paddling
-make a book list and work my way through it. there are plenty of "best of lists" online to help start your own personal list
-find a hobby and participate in a group. model building, carving, gardening (i'd probably go get my master gardener cert because why the hell not?)
-maybe some sort of other activity if it's your thing. theater. choral society. county orchestra. garage band.
-take cooking courses
-get a fun job. roofing? landscape design? driving a forklift*, carpentry
*a friend of mine was walking down the waterfront at 32nd street while in his CO pipeline. He hears someone call his name. Looks around and sees that it's a forklift driver. Realizes that the forklift driver is his former CO from his first tour who went on to make some flavor of SOW O-6. My bud says, "CAPT X, what are you doing here?!" CAPT(ret) X says, "I retired but then got bored and the Mrs told me I couldn't mope around all day. So this looked like fun. Turns out it is."