So you can add on Iceland to any European trip for free, and fly into/out of different cities for less than a return ticket on most other carriers.
10-15 days is a long time for one place (except Japan, where it's a minimum) - consider an 'open jaw' trip where you fly into one city (like Amsterdam) and out of another (Berlin, Paris, London). You can even fly to Europe with IcelandAir via Reykjavík, which includes a stopover in Iceland. So you can add on Iceland to any European trip for free, and fly into/out of different cities for less than a return ticket on most other carriers.