Denise, going to Amsterdam from Stockholm or Copenhagen is a 6 hour train ride. If you plan to go by plane, you need to look into some discount carriers. Plan to spend about 200 pp for airfare. I looked into this for our trip and found it just wasn't logistically or financially feasible...and remember, I live here.

Hamburg and Berlin are more doable. You can use AirBerlin.

I found that one day prior and day of sailing is more than enough for Copenhagen. Copenhagen was just so so.

Stockholm is great and deserves 3-5 days. You may also wish to take a ferry back over to Helsinki and spend more time there. Oslo is doable from Stockholm.