I managed to do the after glow mission in the post game: aside from gaining a weapon that you don't really need, or making the elf girl older, you don't gain shit all. You've to talk to the elf girl and go kill some bosses, and after that you talk to your boss inthe miners workshop, but IMO it's not worth it. The other missions in the post game that i'm aware is the owner scenes, which are quite easy to obtain, pregnancy which are self explanatory and the ultimate boss in the basement of the scholars, you don't gain much or any scene from that either.
Does anyone how to get the threesome and post game scenes for Grus?