Romania

I don't know how much time you have, but these are places I enjoyed on a summer trip. I don't know what they'd be like in the winter.

Bulgaria: Plovdiv and Veliko Tarnovo are attractive, historic cities. In 2015 there were nice, inexpensive handicrafts on offer. VT might be along the rail line for you.

I've read that the border city of Ruse is nice, but I have not been there.

Romania: All the places I visited are beyond Bucharest. I'll list them here because I enjoyed them all: Brasov, Sibiu, Sinaia (for Peles Castle), Cluj-Napoca, Sighisoara and Timisoara. The rural Maramures area in the northwest is extremely interesting, but I'm doubtful about winter conditions there, and it would be way out of your way. I don't know whether any of the cities I've listed would be practical day trips from Bucharest. Rome2Rio suggests that even Brasov would involve over 5 hours on trains or buses, but I haven't checked primary sources for schedule info. This might be a time to consider a private driver. I suspect the cost would be quite reasonable.

At the time of my 2015 visit Balkan trains were quite unreliable. Perhaps that situation has improved a lot, but I'd still recommend checking and re-checking to be sure your trains will actually run.

I would be looking for an individual tour guide for Romania--no matter the season of the year. You can find them on "Romanian private tour guide" in Bing or Google.

I used to have the name of such an individual guide with a car. He would take clients out into the countryside and introduce them to his home town friends--100+ miles north. His customer ratings were tops. Unfortunately I lost his name in a hard drive crash.

I can’t help yet but am looking at Bulgaria Romania in the next couple of years. If you go the guide route, here are a couple that have been recommended by people on the forum.

https://covinnus.com/
https://www.itravelinromania.com/

You may find some more help if you read back through the Romania section. And I hope you will come back with a report and add to the collective knowledge!

In terms of the weather, be well prepared for the cold and dampness. Other than that I would suggest visiting towns in the northwestern region of Romania, ie, Transylvania aside from the capital. Go from Sofia to Bukarest by train.

There's not much in Romania to see during the winter on the way to Bucharest from Bulgaria. Bucharest is only about 75 km from the border and none of Romania's highlights are along the way. A Danube outing could make sense in other seasons, but not winter. So, if you are looking for suggestions en route between Sofia and Bucharest, it would be better to ask for Bulgaria suggestions.

In Romania, Bucharest is worthy of at least a full day on its own. Beyond Bucharest, Christmas and winter conditions are great times to visit Brasov, Sinaia, Sighisoara, Sibiu, Timisoara, Clu-Napoca, and Corvin Castle. The mountains are white and the Christmas ambiance is in the air. But it is very cold. Not as easy to visit the fortified churches with snow and poor weather.

But, only Sinaia and its Peles Castle are really doable from Bucharest as a day trip. I suggest you stick to Bucharest with perhaps a day trip to Sinaia, unless you have time in your schedule for additional Romanian cities. If so, I would prioritize Sinaia(Peles), Brasov, and Sighisoara. If you have more time, Cluj-Napoca and Sibiu can be easily added to that to create a nice circle itinerary. Timisoara is more out of the way - it's much closer to Belgrade than Bucharest. But, it is a great stop on the way to or from Budapest, if you every find yourself with that sort of itinerary.

Corvin Castle is one of Romania's highlights. It is doable as a long day trip from Sibiu or as a pit stop with Timisoara (via Deva & Hunedoara). If you do Cluj and Sibiu, I recommend an en route stop at the fortress and churches in Alba Iulia.

I thought Bran Castle (aka Dracula's Castle) was worth a visit, once you get past all the shops selling Dracula / Vlad the Impaler made in China crap ... not very large but interesting ... Peles Castle was awesome as was the fortified church at Viscri ... the wooden churches in the Maramures region are also well worth it but, as was mentioned, winter time might not be the best time to go seeing as many of these places are off the main roads (which for the most part are pretty bad any time).

So, how was the trip?