I am getting requests to for rental model lately though. Typical one is: "can I have 30 activations for 2 months?" (my customers want to run it on a lot of cloud instances for limited period).
TurboFloat sounds like the perfect fit for these customers. Or, of course, just using regular TurboActivate product keys with custom license fields: http://wyday.com/limelm/help/license-features/
Also see: SaaS and time-limited licensing.
Short answer: yes this is possible in many different ways. How you do it is up to you.
What I can do is just add them to the key and then decrease the count once they expire (I think I am getting them back to use for other keys then, right?). Is there any other (maybe preferred) solution to this problem?
Yeah, you could do that.