Watch some YouTube videos on coloring tutorials. If it looks fun to you, get some adult coloring books and a nice set of pencils. With coloring, it can be as simple as filling in the spaces with color, or you can learn techniques like blending and shading, and bring in other materials like markers/gel pens/watercolors.
YouTube is great for so many things you might want to learn how to do. Maybe some beginner yoga routines, or gentle stretches.
Seize the day lest it seize you. But I don’t think a hobby is what you need. Find a local book club, go bowling, make yourself useful, become the Straight Dope’s most prolific poster, share your life experience with us. Life is wondrous.
I smoke weed and drink beer with our dogs. A neighbor has “borrowed” our dogs and gone hiking.
Horticulture can use massive amounts of time. I could spend an entire weekend pruning, transplanting, thinning out stuff, deadheading, etc without even scratching the surface of what needs done.
Maybe you need more chainsaws.
I laughed too long. I dropped off the big FarmBoss for sharpening.
Besides reading, internet etc. I would suggest you find a Podcast on a topic that interests you and go for walks while you listen to the episodes. That way you exercise the body and the mind 
Thanks everyone. You guys have given me some good ideas. I do walk around my neighborhood, but I think I’ll go for a gym instead. That way I can maybe make some friends at the same time.
I am a 68-year-old woman who has lived alone most of my adult life. Before I retired, I worked three 12-hour days and had four days off every week. I’m used to entertaining myself and have no problem finding things to do at home alone. I have a zillion hobbies that I skip between, and housework and yard work never end.
My suggestion for you is to join ancestry.com and start building your family tree. If you get into this, it can consume a huge amount of time and be enormously fascinating.
Also, take this time to organize your financial / personal / legal information. If you don’t have a will and POA yet, do it now. Make up a list of your bank accounts, credit cards, investments, insurance, important contacts, etc. and let your next of kin know where to find that list.
I am annoyingly bored all the time, with the exception of when it is VERY busy at work, so I am weirdly glad to be very far from retirement. If nothing else works, I would say: go back to work. But first try exercise like others mentioned. I am still bored when I exercise a lot, but I feel great anyway. Unfortunately for me, I had to dial back the exercise, since it makes me feel great all the time. And I reflect that I am becoming delusional, in the sense that there are problems I need to address in my life, but I do not try to correct them if I am high on exercise all the time.
Depending on your financial state, and if you like coffee, find a local diner with a counter, that you like. Start going there in the morning. Sit at the counter. In the beginning you need to go two times a week. Go at the same time on those days. Get to know the waitstaff by name. Do it for a few weeks.
By this point you know every customer that comes in at that time and the waitstaff. It’s conversation time with the regulars and anyone else who comes in. Take it as far as you want.
I don’t follow my own recipe, but I do have my A.M. place to go and Therese and the 8:30 regulars know who I am. I can spend ninety minutes there and pony up $2.20 for coffee and $3 for Ms. Coryell’s excellent service and walk away wide awake and with some socialization.
P.S. I do usually eat a meal too. But many of the regulars do not. They are there for the socialization.
I’m soon to be 61 and while I still work, I ride my horse 4 days a week and go to the gym the other 3. I’m essentially an introvert and would be happy if I never spoke to anyone ever again (except my sister). I have a plan for retirement–volunteer at the equine therapy center. As you might guess, I’m into horses. Find your passion and pursue it.
I thought of a good idea. Play Pokemon go. It gets you out of the house, gives you something to focus on while you’re walking around, and if you live in a fairly populated area you’ll meet other players who you’ll instantly have something in common with.
I know this sounds so two years ago, but my husband and i still play. We go for walks every evening, and on the weekends. There are well over 100 players in our small city alone and we have met some very wonderful people that way.
We are in our 40’s and not the oldest players by far. Most are in their 30-40’s, but the age range is wide.
If you like to sew, own a sewing machine, and have any skill at working with patterns, you could learn to design your own clothes. Or quilt, or anything really.
Go to your local library and check out their selection on arts/crafts, home projects, or whatever might take your fancy and learn something new.