If you've already pointed them to Three Worlds and HPMR and they are interested, they will likely start poking around themselves. So, unless they've asked specifically for suggestions, it isn't clear to me that such suggestions are a good idea.

Assuming they have shown that they are amenable to such suggestions, one slightly Dark Artish but potentially useful idea is to send them first to things they are more likely to agree with. So for example, if they are an agnostic or atheist, sending them to the religion related posts (like the Mysterious Answers sequence) will get them more likely to feel that the website is worth reading. This is because they will be exposed to a combination of arguments they've seen versions of before and ones they have not seen before and all those arguments will likely support their pre-existing viewpoint. This is a very effective way of making people sympathetic and willing to read more.