If you haven't already, get FTL: Faster than Light. I reckon it's one of the best games ever written in any genre and on any platform. I believe it is in the sale.
I just bought Portal 2 and Darkest Dungeon this weekend, both in the sale. I had played Portal before but didn't own a copy, and can confirm that it's still very good. Darkest Dungeon is atmospheric, stressful, frustrating and difficult, but you could perhaps make an argument to your mother that it's educational. You know, in the sense of it teaches younever to go poking about in haunted dungeons if you value your sanityabout risk and reward. If you've played X-COM: Enemy Unknown, it's kind of similar to that. On a conceptual level, I mean.
Incidentally, I don't know if X-COM is in the sale, but that would be another one to try if you haven't already got it. It might look to your mother like a 3rd person shooter, but it's actually a strategy game so you should be ok.
Don't know much about MMOs and I'm not aware of anything that's a clear upgrade on Skyrim, so I'm afraid I can't help you there. Edit: maybe Witcher 3? I haven't played it, but it's got rave reviews.
Also edit: just found this, apparently Recettear: an Item Shop's Tale is on sale. So that's worth picking up. Pit People is also very good, though it's a bit early-access-y. Looks like almost all the Final Fantasy games are 50% off, so it's a good time to explore any of those you haven't gotten around to.