Just been lookin at croatia and slovenia myself as they look spot on and good weather averaging around 29 deg in summer
Ljubljana is a lovely city: small enough to walk around comfortably, nice bars and restaurants, really friendly people. Tallinn is even more beautiful, although it's a bit of a stag place these days.
Someone else mentioned Budapest, probably my favourite city of all, a wonderful place to visit. I've been there eight times and will be there again this summer - a fantastic place.
All of these are considerably cheaper than Paris or Rome.