Well, Sofia is pretty city in its center. There are many nice old buildings, some good museums and art galleries. You must visit The Hram-Pametnik Nevski Church, go for a walk in the City Garden, see the National Palace of Culture, the Opera is worth seeing too. About Nessebar... it is nice in the winter, in the summer is too crowded and is more like big open flea market, not nice. Sunny Beach resort is awful, so are the rest of his type. Places you must see are Kaliakra Cape, Emine Cape, the Kamchia nacional park, and my cities of choice are Sozopol, Varna, and Balchik.

Also Sofia is worth seeing it. But give it more than one day and if you have time visit also the Rila Monastery and Rila Mountains!

Nesebar is nice, however, I'd recommend going even further to the south - Sinemorec is wonderful (or it's how I remember it from my journey 2 years ago). Ropotamo river is worthy to see.

Nesebar still a real place with real people, not so touristic like gold sand for example

Nesebar is definitely worth seeing!

Yep, Nessebar is really good, Sofia can be interesting from some point of view, but the most interesting places in Bulgaria are located near, but outside the cities. The Monasteries, stated below, and many other interesting places, for example Kaliakra.