and get access after activation.
Now i need to perform additional actions right after the activation took
place.

The user_hook as no op for activation, also thought about adding a rule
to the user/reset form but could not find a way to it.

i setup a drupal application where a users can register themselves

Any hints are appreciated.

I am not sure what you want to achieve here. I guess Rules module [1] will help.

[1] http://drupal.org/project/rules