I'm still only partly retired, but my experience runs a bit counter to the responses I've read. I find it challenging to remain intellectually challenged. I had a 20 year military career involving a lot of teaching, learning, and other technical work. I followed this with nearly 20 years as a technical analyst involving heavy intellectual lifting (OK, so my retirement is not as early as others).
Now that I have a lot of time on my hands, I still struggle with keeping my mind going at the same speed as when I was working full time. Sure, there are books, there are courses, there are hobbies, etc. But what I miss the most is the professional interaction with others, playing off each other's ideas. Because of this, I'm still searching for the perfect retirement spot, and it has to include a college with an adult outreach program. But it has to be more than the standard offerings for seniors, I need an active, mentally stimulating, challenging setting. So I may just audit some hard core courses.
At the same time, the setting has to be one of intellectual stimulation without career pressure, so I don't want to be in a classroom with a bunch of young people trying to figure out their lives. I don't know if there is such a setting for retirees, but I'm still searching. I suppose searching is in itself intellectual stimulation. If anyone knows of such a place, I would appreciate knowing about it.