Okay, I’ve got a few here that aren’t really groundbreaking, but haven’t been shared here before for one reason or another - probably because they’re more about Tactics than Key, but whatever.
This one has nothing to do with Key and is pretty obvious, but we can see the cast of MOON. in the One cafeteria.
And in One, Kanon and Air successively, we see the “LOLICON” brand :
Edit: Apparently Makoto is a fan of the LOLICON brand.
Lastly, there are stores named ONE and MOON in the shopping district of One. If you look closely, you can also see a store called “RORIKON”.
Oh, and I almost forgot the Tactics TV in Kouhei’s living room:
Edit: Holy crap, all the other ones I’ve shown here were kind of lame but this one’s actually really cool:
(Minor MOON. and Kanon spoilers)
The two people on the left in the party during Mai’s route of Kanon are Haruka and Ryousuke from MOON.!