I agree that hn fits the bill well. It is quite limited in the diversity of topics and type of people, by design. I've found I also get good mental stimulation

I agree that hn fits the bill well. It is quite limited in the diversity of topics and type of people, by design. I've found I also get good mental stimulation by picking free online courses at random. For example Yale University has a brilliant set of lectures on Milton, which I knew very little about prior.