Edited to expand my comments on Morocco and Ukraine.
Parts of Morocco would have lovely weather at that time of year, but 90 days might be quite a long time there (the experience is quite "foreign").
Also fascinating but with weather probably not as mild as Croatia's, you have Romania and Bulgaria. Weather-wise, Monteenegro's probably pretty close to Croatia, and it is a beautiful country.
Then there's Ukraine. Odessa (the warmest), L'viv and especially Kyiv are worth visiting. The smaller cities are also nice but often don't have tourist offices (nor does Kyiv, as far as I could tell). That means you have to do your own legwork. Public transportation--other than flying--is rather slow and the country is large. Public toilets used at breaks in bus journeys were the hole-in-the-floor type but were always clean and supplied with paper, soap and running water (more than can be said about those in some other countries I have visited).
With the availability of cheap flights I'd try for a combination of the UK (heavy on London with its many indoor attractions) and somewhere warmer, probably Morocco
Remember that you must count both the day you leave Schengen territory and the day you go back to Schengen territory in your Schengen-day total.