Sofia or Ljubljana

If you enjoyed Romania, I say Sofia. But get a guide and get out of town a bit. Plenty for 3.5 days

Ljubljana is definitely more westernized. Sofia is cheaper. I had some difficulty finding a variety of interesting food in Bulgaria back in 2015. I don't typically stay in 4-star hotels or eat in their restaurants. It seemed the inexpensive places I frequented in Bulgaria had somewhat limited menus, and I ended up eating a lot of chicken kebabs. I hadn't done any research on restaurants beforehand, and I didn't have internet access when I was walking around the cities. If you were happy with your food options in Romania, I think the same techniques you used there will work for you in Bulgaria.

I've been to both, but not at that time of year. I really enjoyed the museums in Sofia and I did a fun food tour. That being said, I'm wondering if you're going to be renting a car to get to places? In Bulgaria, you can join some day tour to go to natural places, but since it's off-season, you may be paying a lot for those as the only person on a tour. not sure about that. The food in Sofia was good, the wine was okay. The Rila monastery is beautiful and worth a day trip to. The so-called small villages are really small and many of them are pretty basic. I'm not sure if you just want to see local buildings or what it is about the small villages that you're interested in.
I absolutely love Ljubljana and Slovenia. Once again, are you renting a car or relying on daytours? Ljubljana has a lot of very, very good food and Slovenian wines are exceptional. I went on a food and wine tour of Slovenia and was in heaven. Lake Bled, though touristy in season, is always worth going to, even just to walk around the lake. Gorgeous area. Once again, I'm not sure what the weather is like is March - if it's still snowy. But there are many places to hike in the Julian Alps. The western part has lots of small wineries to visit and lovely countryside. Ptuj, in the east, is another good base for drives around the eastern area. You can PM me with specific questions if you'd like.

Slovenia is the crossroads of Italy, Austria, and the rest of Yugoslavia. As such, it is the best of all IMHO. We've been 2x. I'd go again tomorrow. Ljubljana is a very nice small capital city.

Ljubljana was a great surprise for us. It was vibrant and fun. We only planned 1 night as a stopover and wish we had spent another. Lake Bled is just a short drive away and is a good day trip. I've yet to visit Sofia but it does appeal to me. You'll enjoy your trip wherever you go!

It depends on what are you looking for. Personally, I love both cities and think both are some of the most underrated cities in Europe but if you have to choose I'd say this; If you're seeking a vibrant nightlife scene with an eclectic mix of bars, clubs, and live music venues, Sofia is the place to be. The city comes alive after dark, offering a wide range of entertainment options for night owls.

On the other hand, if you have a penchant for stunning architecture and picturesque old towns, Ljubljana is a true gem. Its historic center is a real-life fairy tale, with charming bridges, colorful buildings, and cozy cafes that create a romantic atmosphere perfect for leisurely strolls and architectural exploration.