Barcelona is a great city, lots to do and see.
Madrid is great perhaps less popular than Barcelona.
Rome, excellent lots to do.
Oslo a fabululous place, VERY expensive however, more so than say Stockholm or other Scandinavian places.
Fes a really interesting place to go or the more popular Marrakech.
Budapest is a great city to visit as well.
Palma is also an interesting place to visit, flights are very cheap and you can visit the city and drive a little around the island too. A good mixture.
Moscow is really interesting too. June a good time to go!
I travelled a lot before the children arrived. Some of these places are great with children too!
Depends how long you want to fly for!