You want authentic? A fun local thing to do in Ljubljana is to get a late night "hot horse burger" from one of the street stands. Yes, it's horse meat, and they're very tasty. Of the cities you listed, it's the lowest key by far, especially during summer tourist season. Of the cities you list, it's the closest to nature as well. Drive into the countryside and visit Lake Bled or Postojna Cave.
London, Rome, Berlin....the possibilities are endless. It's hard to recommend anything without knowing what your interests are. One thing I always recommend to guys coming to London is to check out the men's stores. There is probably no where else in the world with such a concentration of retail devoted to men in one small but often missed area. It's interesting if nothing else. From Piccadilly Circus/Regent Street, check out the length of Jermyn Street (the street is almost exclusively men's stores of all kinds: shirts, shaving, shoes, hats, accessories), then go through the Piccadilly Arcade (shops) that end on Piccadilly (the street), then cross the street to Burlington Arcade (shops) and go through it that end on Vigo, turn right and then couple streets is Savile Row on the left.
I'm also not a fan of exclusively using the Tube. You miss out on the street vibe of London. Sure, use it to save time, but throw in a few bus routes and taxis rides to get a sense of the city passing through on top.
Have a good time!