I've been retired for 5 years now and like you I started getting bored after awhile.
But for me it was just a matter of adjusting to not being busy all the time. You don't have to stay busy to keep sane.
Sometimes it's nice to take it slow.
I do find things to do around the house, I'm handy with fixing things. There's always some repair that needs done whether on my house or my car, lawn mower, etc.
I also keep sane by getting out and riding my bicycle on nice days, or even just take a walk.
Doing my own yard work such as mowing the lawn, landscaping etc keeps me busy.
I find I don't have the time to get everything done and there's always more things to do. But if I don't feel like doing something one day then I'm okay with just lounging in my recliner watching television or browsing the internet on my laptop.