My suggestion isn't quite going to be a link, but here it goes. Get a hobby. Programming, sport, (specific) art, etc. There is a myriad of niche communities for all sorts of things, and I would argue that the more in-depth creative ones are more stimulating than any of the "general-purpose" ones listed here.

My "intellectual stimulation" consists of coming up with or doing things. Sure, self-reflection helps, but more often than not, they're inspired by something I see in a "specialized" community.

Slightly off-topic, but asking "why" helps with self-reflection and creativity.