If Plitvice Lakes National Park usn't already covered, that would be my top recommendation. It really needs a night at a local lodging before your visit so you don't have to contend with thousands of day-trippers sharing the walkways. Time in the park can be under 6 hours.
I like Istria, Ljubljana and Bled, which are quite different from each other (and Ljubljana's different from Zagreb), so it sort of depends on what you think you'll be be interested in seeing by that point in your trip. Use ViaMichelin to look at possible driving routes and estimated trip times, but note that folks here feel that VM's time estimates are optimistic.
Some of the key spots in Istria are: Pula (probably only for the Roman ruins), Porec, Rovinj, Motovun (inland), Groznjan (inland) and Piran (Slovenian Istria). I don't know how tight the hotel situation is in the other places, but from personal experience I suggest pre-booking Rovinj if you want to spend the night there. And I assume a last-minute search around Plitvice would also be risky.