Slovenia/Istria for 10 days....need one more day trip idea...

Even though it's not exactly what you were looking for, I will note what a lot of
people comment on when posters ask for feedback on their itineraries; just make
sure you factor in travel times when you put together your plans.

A different place every day, assuming you're traveling by bus or car since trains
aren't very efficient in that part of the world, means you're getting maybe a half
a day and an evening in each place. The drive from Triglav to Rovinj seems
like the better part of a day all by itself, and this is not really taking into account
stopping to see things along the way.

Also, you have Postojna Caves on one day and Predjama Castle on another, but
they're right next to each other. Since day 5 and day 8 are your longest travel
days, do you have enough time to squeeze in those sights (especially with a
timed entry ticket for the caves)?

I would say that you probably have too much in here already, and that you might
consider adding a bit more time into what you already have, and maybe even
taking something like Pula out.

I probably should have mentioned I'm staying at airbnb's in Ljubljana for days 1-4, Rovinj for days 5-7, and Ljubljana again for days 8-10, so the driving times aren't as bad as you'd think.

Hello from Shaker Heights :) We had a wonderful trip to Slovenia last year, see my Slovenia Trip Report (22 days).

Have you thought about seeing Triglav and Soča Valley on your way to Istria?
Days 1-4: Stay in Ljubljana; see old town, Lake Bled, etc.
Day 5: Stay in Soča Valley; after driving through and seeing Triglav
Day 6-8: Stay in Rovinj; see Piran, Koper, Pula, wineries, etc.
Day 9: Stay in Kranj or Ljubljana; see Postonjna and Predjama on the way
Day 10: fly home

Depending on your flight time, you might consider driving to the airport on Day 10. We left from Vipava the morning of a 12 pm departure with plenty of time to return the car before checking in for our flight.

There is a lot of back and forth in your itinerary that doesn't make sense to me.

Your itinerary with my comments in ():
Day 1: arrive in Ljubljana, explore old town (yes)
Day 2: Ljubljana old town and vicinity (yes)
Day 3: Lake Bled (yes)
Day 4 Triglavski (as a day trip? No, too much backtracking)
Day 5 Travel to Rovinj, stop at Postojna Caves on the way (why would you stop at the caves on the way when you went by them the day before?)
Day 6: Piran and Koper (Piran deserves an afternoon and Koper is not interesting)
Day 7: Pula and area wineries (the wineries are nearly 2 hours in the opposite direction of Pula)
Day 8: Back to Ljubljana, stop at Predjama Castle on the way (the castle is right by the caves, so couple them)
Day 9: ????? Zagreb? Maridor? Ptuj? Something a local Slovenian knows about that I don't? (Zagreb, no; Maribor, no: Ptuj, no; just spend more time in Istria)
Day 10: fly home

My suggestion:

Day 1-3 make sense.
Day 4 - go to Soca Valley and stay two nights.
Day 5 - Soca Valley
Day 6 - Go to Rovinj and stop at Piran on the way (skip Koper, not interesting)
Day 7 - Istria (Pula, Kamenjak)
Day 8 - Istria (wineries, hill towns)
Day 9 - Drive back to Ljubljana and stop at the caves and castle on the way
Day 10 - fly home.

Day 9: I'm not Slovenian but visited all 3 and Ptuj Castle was the most interesting for us.

I would agree to skip Pula; we were there last week and underwhelmed. On the other hand, the hilltop villages north of Rovinj are lovely to stroll through, and there are many vineyards in the area. We enjoyed MARE, a small family owned vineyard in Rovinj. The owners husband is also an owner in Benvenuti Winery (Kaldir, 52424, Motovun, Croatia), which we sadly didn't have time to try. We also enjoyed Cossetto (Roškići 10, 52464, Kaštelir, Croatia). We didn't need a reservation for MARE, but you will need for the other two. We also couldn't get a reservation for Rossi Winery, which produce DOC wines.
If you don't have lodging, you might look around Rossi; there were several lovely properties in the area, Motovun or Grožnjan. We enjoyed Rovinj, however it wasn't as scenic or special as these two towns.