I just returned from Bulgaria today after a 3 week driving trip. Roads are not great but doable. Sofia Airport is tiny, no great delays at any time and 15 mins from the city. Intercontinental a good choice, comfy beds and close to main street and sttractions and the Sofia free walking food tour and other tour meet up points. 1.5 days is enough. If you are fit, add a day to do the 7 lakes and Rila monestary tour. Worth the walk. About 8km loop, half uphill to get your view of all 7 lakes and a great chair lift ride.Pack your own lunch and outdoor gesr for wet or cold changes.

I noted Varna had direct flights to Istanbul, but I would go there for sunny climate not winter time. I think Plovdiv would be my Bulgarian, Turkey side trip choice.

Varna is amazing for a city beach destinstuon, stay in the newly renovate Cherno More Casino and you can walk down and hire a sun bed and est st the beach clubs in 600m walkimg and also enjoy the gsrdens, Catherdrals, old town parts and a communist car retro museum at Grand Mall.

Plovdiv is the best if you like dining, walking, Roman ruins and the airport is also close to town, free walking tours, stay in the pedestrian only area off the Mall or Kapana district and enjoy a really nice city vibe with lots of locals and visitors, also has really accessible roman ruins. Plovdiv much hotter than Sofia so we booked to stay 6km out of city at Landmark Hotel which has a pool. About 7 Lev for a taxi to town.

I could easily have skipped Sveti Vlas, Sunny Beach, Thracian Cliffs, Balchik and Golden beach is nice, but try to stay beachfront. Some hotels are way up the hill. Ditto for Sveti Vlas if you ever head there.