A trip through western Europe and Scandinavia

I have two weeks time in the beginning of September to do some solo travelling. In Europe, I've already been to:

• London
• Paris
• Barcelona
• Rome
• Pisa
• Verona
• Florence
• Venice
• Salzburg
• Vienna
• Prague
• Heidelberg
• Frankfurt

So far, the cities I really want to go are Berlin, Amsterdam, and Stockholm. I want to stay within Western Europe and Scandinavia. I'm interested in cultural stuff, beautiful scenery, meeting people and shopping. Can you guys give me some other suggestions on what to do in those cities, and any other cities that's worth visiting? Thanks!

Just want to add I'm leaving from Frankfurt, and must come back at the end of the two weeks. Would also like to go to Switzerland, but not sure where. Possibly Hamburg too (is it worth going?). What's a reasonable number of cities to visit within 2 weeks?