Is it worth going to piran for a few hours?

I have not been to Trieste, but looking at pictures online, it looks like a bigger city and not very similar to Piran. So if you will not be visiting any other Adriatic coastal towns, I think Piran is worth a few hours to wander around, have some lunch or a drink on the main square.

I don't know what it will cost you to have your driver detour you there and wait for you, but if the price is reasonable and you have the time, go for it.

I will visit muggia which is a small coastal town closer to Trieste.
Another option would be to stop at a medieval town on the way.

Piran is a lovely town but is fairly small, so it can be explored quickly. You do have to park in a lot just outside the old town and walk in, though. The center is restricted to residents or temporary passage for taxis, I guess.

If you are interested in other options, my suggestion would be to go from Bled to Trieste via Kranjska Gora and over Vrsic Pass. You could then stop in Kobarid and visit the WWI museum there if that interests you, or you could stop at Tolmin Gorge, or you could detour to Skocjan Caves.

Muggia has a nice historical center, but it is absolutely tiny, much smaller than Piran's already small size. If you can do the detour at a reasonable cost, go for it. I also like Lane's suggestion, very scenic.

Which medieval town along the way were you considering?

Skofja Loka and radovljca were two and I considered staying in radovljca instead of bled.

We won't have a car but a willing to take taxis if the destination is special.

I'm not interested in hikes or caves but walks are of interest.

Vintgar Gorge is nice, but don't expect to be alone there.

I had no such expectations. I've been other gorges including ones in the Rockies. It looks fairly easy,I'm fearful of height.
Just wondering if it's really special. Nice isn't an encouraging adjective☺

I think "nice" is a good description of the Vintgar Gorge hike. It was very pretty and in spots dramatic, with a nice waterfall at the end, but, to me, not amazing.

I think it depends partly on your background. I'm from NC (mountain streams, etc.), and not all that many years ago I went to a gorge near Lake Placid, NY, that was somewhat like Vintgar. I enjoyed both and think Vintgar is definitely worthwhile if you're in Bled and have a bit of extra time. But it's quite developed and popular. As for heights, the walkways at Vintgar are not all right down at water level, but they are not very high, and I think you'd be unlikely to have issues with them. You can probably find some videos on YouTube if you want to investigate further.

Special is a difficult adjective as it's very subjective. Personally, I would say that if you are in Bled and have the time and opportunity, go visit the Vintgar. There are other more impressive sights in the Julian Alps, but if you don't want to hike and have a fear of heights, you will have to stick to lower lying places of interest. If you had a car, I would suggest a drive over Vrsic Pass into the Soca Valley, but it's a bit difficult by taxi, not to mention your time constraint. Bohinj is another nice area to visit but it deserves a full day.

Radovljica is a cute town and a place to consider visiting if you drop the Vintgar. Skofja Loka can be visited on the way to Ljubljana.

We took an Arriva bus from Ljubjlana to Trieste this past March, was about 2 hrs. After seeing a lot of Trieste over several days we took a direct bus to Piran for a few hrs. And lunch. Pretty drive once got to the Slovenian area. On the way back you need to catch a bus to Koper, about 1 euro as I remember, which drops you off at the bus station and then we bought the ticket back and waited about 30 minutes for the Arriva bus back to Trieste.

There is a cave in the karst at the top of Trieste, Grotta Gigante, which was fun to visit. You have a 1 hr guided tour. Our guide answered any and all questions. We got a brochure at the TI in Trieste as wanted something non museum and non church related at that point.