Skip the expensive fast boats and take the regular IDO ferry to the Princes' Islands. The slow ferry from Kabataş takes 60-80 minutes but that's the whole point - you get to watch Istanbul slowly disappear behind you while seagulls follow the boat.

The fast catamaran costs 3x more and you miss all the scenery. Plus the slow ferry has a proper café with Turkish tea and simit. Standard ferry fares versus premium express prices. Sometimes the best part of travel is not rushing.