6 days in Slovenia Itinerary

I don't understand why you are going back to Ljubljana on Day 3 and doing day trips from there for the rest of your time. Why not stay in the Soca Valley (Kobarid maybe) for a few days?

Piran is very nice, but I don't think it's worth a day trip just to go there. I stopped there on the way from Kobarid to Rovinj, enjoyed it for a couple of hours. You could combine it with a stop at Skocjan or Postojna caves and Predjama Castle.

Where in Croatia are you heading on Day 7? And how are you getting there? If it works logistically, you could stay three nights in Ljubljana at the beginning and then three nights in Kobarid and accomplish the same objectives.

Well I just check all those different cities(Piran and Soca Valley), they are just 1 1/2 hours away in Ljubljana.

Are you flying into Ljubljana? If so, take a few days to explore the city. We did it in 2.

Slovenia is a small country and we did a loop tour.Staying in lots of places instead of going back to Ljubliana every day.
Ljubliana(spent 2 nights)-Lake Bled(spent a night)-Triglav and Soca Valley-Kosec(spent a night)-Piran(spent a night) and then onto Croatia.

Your Lake Bled is doable. We did the Gorge in the morning and walked Lake Bled in the afternoon. I wish we would have had another day exploring there. WE could of explored the castle area more and slowed down a little. We stayed the night and moved onto Triglav the next day. If I was to go back I could easily spend all my time in Triglav and the Soca Valley. It was amazing. The highlight was the famr stay in Kosec. The food was amazing.

When we booked our 2 weeks trip Slovenia was kind of an afterthought to Croatia, but I wish I would have spend about 3/4 of my time in Slovenia and just a little in Croatia.

No i'm coming from Vienna. Then i will arrive at Ljubljana in the morning. Did you go to Ljublana in the morning to go to Croatia or you just travel to Croatia from Piran?

No i'm coming from Vienna. Then i will arrive at Ljubljana in the morning. Did you go to Ljublana in the morning to go to Croatia or you just travel to Croatia from Piran?

Have you looked at a map? Prian is on the border with Croatia - where in Croatia are you heading to?

When we booked our 2 weeks trip Slovenia was kind of an afterthought
to Croatia, but I wish I would have spend about 3/4 of my time in
Slovenia and just a little in Croatia.

I'm an obsessive planner. When I planned my 2 week trip to Croatia and Slovenia, I discovered that Slovenia had a ridiculous amount of things to see and ended up converting the 2-week Croatia/Slovenia trip into a 25-day Slovenia trip (western Slovenia, actually). The trip report is here: https://community.ricksteves.com/travel-forum/trip-reports/25-days-in-slovenia

Should I add more days?

The answer depends on your travel goals. I think 6-7 days is pretty close to the standard "hit the big sites" Slovenia trip. It seems to satisfy many, but it leaves others sad they did not spend more time there (see the post above). I travel at a slower speed, so it would not satisfy me, but there certainly is nothing superior about the way I travel -- it just works for me.

What should I do differently?

I've been thinking about your question since this post went up. I'm always a little hesitant to give advice to someone who is approaching their trip in a way different from my usual approach, but here goes...

I think one thing to do is to figure out how Slovenia fits into your overall trip. It seems like Slovenia is your chance for mountain grandeur, so maybe that's a good place to focus your time, along with some time in Ljubljana, which is a great city.

I agree with Lane that more than one base would be ideal and if you stick with your current 6-night plan for the country, something like jrdmcbroom's point-to-point travel plan would be something to consider.

I would also suggest considering how getting in and out of the country impacts your itinerary, too. You don't mention how you are planning to travel between countries. If you are arriving from Vienna by train, entry at Bled works as well as entry at Ljubljana. Heck, even if you are arriving at the airport, going to Bled is as easy as going to Ljublana.

You currently appear to be planning to exit Slovenia from Ljubljana to go to Split, which is a super long travel day, Ljubljana to Plitvice Lakes makes more sense geographically. You could catch a train to Zagreb and pick up a rental car there and then make the drive to Plitvice Lakes, doing the recommended afternoon/morning visit to try to minimize visiting the park at its most crowded times. Then you could move on from there without back-tracking (Krka National Park, Split, Dubrovnik).

So... maybe it makes sense to start at Bled and end at Ljubljana?

As for Piran, it's a cute town on the Adriatic Sea. I love it, but I don't know that it adds that much to a trip that includes other towns/cities on the Adriatic Sea. Given your plans for Croatia, I would probably skip it, especially if you are not planning to spend the night there; it's more atmospheric in the late afternoon/evening.

I think If you keep your current amount of time in Slovenia, I would think about an itinerary that looks something like:

Day 1 -- Arrive Bled
Day 2 -- Vintgar Gorge at opening time to avoid crowds and then more Bled OR short drive to Lake Bohinj OR visit the cute old town of nearby Radovljica OR ask Tina/Sašo to show you something cool near Bled
Day 3 -- Drive Vršič Pass -- spend the night in Kobarid (or better yet at Tourist Farm Kranjc at Koseč if they do one-night stays) OR catch the Motorail (car train) at Most na Soči back to the Bled area if you don't want to make a hotel change
Day 4 -- Spend an extra day in Kobarid to see the excellent Kobarid Museum and/or Tolmin Gorges OR schedule a tour with the excellent guide (and herbalist) Vesna OR stop at the UNESCO World Heritage mercury mine at Idrija and/or stop at the Franja Partisan Hospital on the way to Ljubljana
Day 5 -- Ljubljana
Day 6 -- Ljubljana

I think Škocjan Caves is something very special, and it is also a Unesco World Heritage site (not that UNESCO sites guide my travel, but it gives a sense that others consider it something special, too). It could be done as a day trip from Ljubljana with a stop at Predjama Castle, as Lane notes above. If you have an extra day to throw at Slovenia (and nature is in your realm of interests), it would be a worthy addition to your list. In the largest cavern, you will find yourself wanting to shout, "You shall not pass!" like Gandalf. Or at least I did.

Thank you all so much! I appreciate all the replies and it really help my itinerary in Slovenia. But I have more question, is it better to stay in Kobarid and travel to Piran or I should visit Piran coming from Ljubljana? Because when I search which is nearest from Piran, Ljubljana is near in Piran than Kobarid by 30 mins.

The trip from Ljubljana to Piran is nearly all highway. The trip from Kobarid to Piran has some highway, but is some 2-lane roads that go through small towns, too. If you are doing a day trip straight to Piran and back, Ljubljana makes sense. For stops along the way, there is probably more of interest coming from Ljubljana (unless nature is your top interest). Obviously, you could also hit Piran traveling between Ljubljana and Kobarid.

Happy travels!