By adding comments events to the main page, I got rid of the activity page. Let's say I merged both.
The only difference now, as activity page is gone, is that users cannot see "edit" and "retagged" events any more. And I think they do not care about.
However, big advantage:
→ Main page shows questions on top that got request as comments.
→ This helps askers that need another answer to bring their question on top again.
Btw, do members really check the /activity/ page? I am too lazy. Check your server stats :)
Core hack implemented in http://www.gute-mathe-fragen.de/