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 
From personal experience, I say that the best way to earn STEEM is by playing SteemMonsters. I only have 60 SP and if I lease it will only give me a daily refund of around $ 0.008 and if I play SteemMonsters with an initial account, I can get daily cards with a value of $ 0.01 ( daily quests ).SP 60 = $ 20 and I receive $ 0.008 per day or I buy the initial package in Steem Monsters ($ 10) and receipt of $ 0.01 to $ 0.15 (minimum) per day and every 14 days $ 0.14 to $ 1.14 for the season rewards ( minimum ) . Without counting other advantages that are obtained in Steemit and if you are a good player you can win STEEM in the tournaments.
I play only 30 minutes per day
Now if you only want to get STEEM through your SP, you can use SmartSteem or MinnowBooster to delegate your SP or sell your Upvotes