What services which allow to multiply our STEEM (Steem Power/ by leasing etc) are the most attractive now?
What are the ways to earn additional STEEM at all? I heard about services which allow to delegate votes or so - I am looking for the best way to allow my SPs to not lay useless when I am not using steem

Delegate to a bid bot such as smartsteem or upmewhale and they will send your share of the bids daily.
That may not be the highest return possible (and probably isn't) but it is definitely the easiest by far.
You can find a list of operating bid bots here:
https://steembottracker.com/Isn't there a more ethical way to earn rewards on our stake?
Not that I'm completely against bid bots, but I'd rather invest in something that benefits Steem overall.
Also, surprised to see you here, didn't know you were an active Bitcointalk user
Sure, you can curate yourself or delegate to a curator and agree to split the rewards somehow. However, earnings will be a lot less than a bid bot. Alternately you can try to find a paid lease deal (eg. via dlease) where the lessee isn't just looking to self-vote or beef up their bid bot. Not necessarily easy, but perhaps possible.
Unfortunately, the incentives on the Steem blockchain are pretty broken, so when someone asks what is the best way to earn a return and someone asks how to benefit Steem the answers are a lot more different than I would like. I'd like to think that the incentives can be reworked and and better aligned, and that this can happen relatively soon, but there is no way to know for sure whether or when that will actually happen.