I'm not retired or financially independent by any stretch of the imagination but something similar happened to me once I decided to work from home in business for myself. Since my work is still fairly sparse my situation is probably similar to someone who has retired, except of course I lack the financial independence. It'll be 18 years this September since I've been doing this. I'd say 45 weeks out of the year I have literally nothing work related to occupy me (I'd like to get more work once I get the house in order, but that's another story). Anyway, I can honestly say I'm not bored yet. My life might be a little more interesting if I had more disposable income (even taking the subway into Manhattan is something I can't do as often as I'd like) but I can hardly say I spend days not knowing what to do with my time.
I too thought I might get lazy but that hasn't really happened. I keep my mind active, I keep my body somewhat active (the Internet unfortunately hurts those efforts). I enjoy not having to run out the door at the crack of dawn each day and then return home long after the sun has set with barely enough energy to eat dinner and perhaps shower, then go to sleep dead tired only to repeat this ritual four or five more times that week. I especially enjoy not having to go out during the summer months, which was the very time I hated working the most. I enjoy knowing my fate is in my own hands now. And I most definitely enjoy not having to waste hours on the phone with customers answering the same questions for the same people week after week.
I thought the social isolation might bother me but to some extent forums like this help. I also realized that interaction with coworkers was highly overrated anyway. Most places I worked I saw the same five or dozen people day in and day out. It wasn't stimulating at all. In fact, more often than not they got on my nerves more than family members. The customers I interacted with were worse.
As strange as it sounds, the only thing I do miss a bit is getting to work. Every day you see different people on the subway. Sometimes you even have random conversations with them, whether for one minute or half an hour. It was always an unknown who you might see on the train that day. I talked with all ages at one time or another. Perhaps one day it was an elderly man, another maybe a middle-aged women, yet another perhaps a student in high school or college or even grade school. It was always interesting. Rarely did I initiate the conversations but always enjoyed them once they started. These subway rides were the only thing in my life different each day. It was possible I could meet an old friend, or a teacher, or even my future wife on the train. I really miss that part of working but not the rest.
I've always been a fan of the unknown. Now that I'm in charge of my life, each day is exactly that. No set schedule when I'll be eating, sleeping, on CPF, working in my lab. I like this lifestyle much more than the routine of a job. In fact, I often resent getting work now as it takes me away from this, but I haven't reached the point where I can afford to turn away work. So long as you're a self-starter, you'll have no problems finding something to do.
That's all I can think of for now. If you've really reached a point in your life where you can not work at all and live comfortably, then congratulations. I hope to get there in time as well. Even though I must still take any work I get, and will actively seek more in time, I can honestly say getting away from the daily grind has probably added decades to my life. I knew things were seriously wrong when I was screaming at my alarm clock every Monday morning. The life I was leading before wasn't a life to me, it was an existence. I felt like a rat on a treadmill.