I am planning a (mostly solo) backpacking trip next Oct in Europe for about 4 weeks. It will be my first visit there, I am looking for advice on which cities to visit. I like go to go cities with architecture, history, and different life style (compare to America). I prefer walking in a local market and interact with locals over drinking in a bar/pub. I have pargue, berlin, vienna, luxembourg, ambsterdam, paris on my reference list. My question is, what (other) cities are worth going?

I have some UA miles for flight tickets. I assume I should fly to frankfurt or a major hub and do a loop around europe? Is there a better way (open jaw?). For accommodation I am looking at hostels or something from airbnb. Since I am mostly traveling alone, safety is my # 1 concern.