For city viewing, pre and post season is great choice
Though please do watch fo weather they did announce that next week sould start with rain and temperature drop

If you’re plannig to visit Rovinj close by are Brijuni islands with multitude of stuff to see:

• Roman villa and thermal baths
• Byzantine castrum
• Dino footprints

Larger island is also a NP with some unexpected animals to see: Brijuni’s fauna