Budapest is a chill city. A couple of years back, myself and a good mate spent a month travelling around Eastern/central Europe, half the time we had pre-booked hostels, but in cities I had been to before we kipped in sleeping bags. Budapest was one of such; there is a hill/park in the city centre, right on the Danube (on the Buda side iirc). Sleeping on the top of that, smoking some nice weed from our newly-aquired Slovakian pipes, under a blanket of stars amd city lights, during one of those balmy summer European nights.
Romania is nice as well, though I wouldn't feel safe sleeping rough there, at least not in a big city. During the same trip, we arrived in Bucharest by train. Before finding our hostel, we planned to get a feel for the city centre. Asking a taxi driver to take us there turned out to be problematic, he dropped us off at some shopping centre, arbitarily called 'city centre'. Shrugging our shoulders we had a jay in a park and went for a wander, only to stumble upon an abandoned power station which we happily explored. Ah, surreal stoned dazes.
Bratislava is another nice city, though staying in a hostel there after seeing the film of the same name is a little daunting!
EDIT: Skegness, thee town of lights and laughter!