If Krakow Poland is "freezing" in May, avg temp 18 C , don't bother thinking of ever going there, as year highest avg is 22 C in July and Aug.

My suggestion is, as many others, Lisbon. You seem to be worried about the hills - no need. I took my then 84 year old dad to this city. We travelled by clean, efficient and cheap subway, cheap taxies, and in the old town, when needed, took the street car up the hill. All very convenient. ^

As the price level seems to be a major influence, my opinion is that Lisbon has the lowest combined cost (hotel/transport/restaurant) compared with all western European capitals. ^

I've so far no experience of Porto, of which I heard much good, but will have after a planned visit in June or July.