Is there any good way to create a list of all the editors in a site and see if they've published or edited anything lately? Or do I have to loop through every page on the site and check all versions?
You can easily find the last login date of your editors, not sure if that´s enough?
No, that's unfortunately not enough.
I have an editor-group and from that I'm getting a list of users from the ad. I want see if they've made any updates or published any new articles in the last couple of months.
If you are using EPiServer 6 R2 this might be interesting: http://world.episerver.com/Documentation/Items/Tech-Notes/EPiServer-CMS-6/EPiServer-CMS-60/Change-Log/
Seems like you don't need R2 btw. Just EPi 6.