Surfing was one of our top causes of workplace absenteeism illness so, not needing any more temptation in my working life, I waited until retirement to learn.
Afternoon naps. This is a lot easier at home than in the office. An unexpected side benefit has been that it's a great barometer of whether I'm feeling healthy or not... somedays I confuse fatigue with eyestrain and a nap sorts that right out.
I used to bicycle 2000-3000 commuter miles annually. I haven't had the bike out of the garage in weeks. I'd rather spend the time surfing or working around the house/yard.
I used to swim laps & run, but that's been replaced by surfing and walking. Spouse & I especially enjoy after-dinner walks (and our kid enjoys being left alone in the house).
I used to spend a lot of time in the gym hefting weights, now I spend it in the back yard hefting compost. I'm getting to be a much better landscaper/gardener (thanks to my father-in-law and the UH Agricultural Extension's website). Ironically it has an ROI since our fruit production is way up. In that light, our next crop will probably be tomatoes.
Home improvement skills have also significantly, uhm, improved. Part of that has been more time for "This Old House" and Family Handyman magazine.
I spend a 2-3 hours more each day reading-- both in print and on the internet. I'm probably one of the library's biggest customers. Even if you account for the shift from nuclear-power manuals to financial management I'm still reading more.
When I was working, I thought that retirement would involve more volunteer work and catching up on TV/movies that I've missed over the last couple decades. I've been too busy to attempt either of those.
My kid led me to tae kwon do, and I wish I'd found it 20 years ago. Part of its enjoyment comes from a good instructor and a good crowd of fellow students, but it's really paying off in agility, balance, reflexes, and weight loss. I'd highly recommend some sort of martial arts for anyone at any age.
One caution on physical activity in retirement-- it can cause a lot of pain (even at age 44). I'm in the best shape of my life but when I pull my head out of an activity I usually realize that I've overdone it and I'll be paying heartily despite 800 mg of ibuprofen. Tae kwon do, surfing, yard work, even housework have all over-exerted muscles that I didn't even know I had. Part of the problem is that in retirement I don't have to quit after 30 minutes or an hour-- but instead can keep going until the job is done.
Now that we have plenty of time to travel, we don't seem to be that interested. That may change when we're empty nesters, and another part of it has been pet/home care. Or maybe it's because we can travel the world in video without un-wedging our butts from our recliner.
When I was working, I only had time for a couple of interests. I thought that in retirement I'd develop plenty of them. While I've added a lot of things to my "Try before you die" list, I've spent a lot more time thoroughly exploring one or two interests instead of shotgunning the field. There just doesn't seem to be any of that old "Get this done before you have to go back to work" urgency.