First vacation alone: Lisbon, Valencia or Krakow?

I am 36 years old and so far I have always traveled with friends or partners. In October I would like to leave alone for five nights, choosing a city easy to turn, with quiet hostels or small hotels and opportunities to meet people without having to make nightlife. Lisbon attracts me for the atmosphere, Valencia for the sea and Krakow for costs and dimensions. My biggest concern is to feel isolated at dinner and return late in a little busy area. I am interested in a comparison based on recent experiences: indicat…