Just back from Croatia. You can check future events on any city's tourist office site. Rovinj has a good one. May ls start up time, and people are rested and not overwhelmed after a summer of overtourism - like this one, strangely. Restaurants take 6 month licences from May to September usually. Local restaurants stay open all year , worth finding, not all on the Web.
Museums in Zagreb may still be closed for repair and renovations, as building season is Sep 15th to June 15th and work is going on to repair the earthquake damage, so check each museum you want to visit. Great views from the top of the Tower above the Funicular mini train up to upper town, both worth buying a ticket for if not afraid of heights or spiral stairs. Cafes all open as usual.
As my route via Venice was off - no ferries ran this year - I flew Lufthansa/Croatia Airlines via Frankfurt to Zagreb, simple.
The coast and the south have better weather than continental Zagreb if you want to guarantee warm rain or sun.
10 days is not really enough to get over jet lag, consider a few extra days if you can. As the first guests of the season you may be seen as good omens and treated even better by courteous Croatians. Check all Covid protocols and follow them. The Enter Croatia online application is one of the simplest in Europe if vaccinated. and can be done weeks in advance. A Zagreb airport officer just swiped my passport, let me in. There is a friendly Welcome to Zagreb counter if you need any help - my Croatian SIM had expired.