Thea:
For a third base, consider Veliko Turnovo, the ancient capital of Bulgaria. It is far more colorful and interesting than either Sofia or Plovdiv, IME, IMO. I would also recommend Varna on the Black Sea, not for swimming, but for a picturesque, larger city whose Marine Gardens are outstanding.

My first visit to Bulgaria was in 1980's and most recently in 2005. Both times I was in VT doing some research for a book. VT stood out as the highlight. I recommend that you consider driving. Roads are uncrowded and quite well maintained and the motorways are excellent.

Western Europeans have discovered the country as a skiing mecca mostly, and the only American I've ever met in the country (Sofia) was a Univ of Wyoming Professor teaching English. He introduced us to a wonderful night club dancing show high on a hill overlooking the city ("Boiana").

If you drive, you can stop in the restored "museum village" of Koprivshtitsa, between Sofia and VT. A suburb of VT also is a "museum" village (Arbanasi) with beautiful and stately old homes. Baldwin the Crusader's castle is a major site in VT. The massive Valley of the Roses is between Plovdiv and VT, eastward toward Kazanluk...season is May/June. I've missed it as my visits have all been in fall.

I will be happy to recommend lodgings and restos once you firm up your itinerary, along with pertinent pix.

Thea, you're in for an adventurous visit to a scenic country.

Stu Tower

I havent done alot of Bulgaria, but the areas I have done were very good. Plovdiv is a lovely city and very inepensive. I have also hiked in the Rhodopa mountains with their excellent villages. The people are lovely .... very friendly and generous.

I agree with Stu that VT is a good location. But I would also look at spending some time in the countryside, I didn't, and regret it, but I think it may be difficult by public transprt. Click on my name for my TR, or start here for text with pix: http://mytimetotravel.wordpress.com/2011/10/11/time-to-tour-in-sofia/

Thea: This will get you started. Some are pre-digital scanned, and others are digital, covering the 1980's and most recent 2005. They take in Veliko Turnovo, Varna, Koprivshtitsa and Sofia.
Stu
https://picasaweb.google.com/stuarttower/Bulgaria1980SAnd2005#

Thea..update on Bulgarian pics, more of Varna and VT. We were "invited" to a wedding in VT and my late wife led some of the folk dancing with which she was very familiar. They couldn't believe this little American woman knew those dances.
The bride's uncle had seen us standing by our rental car (German plates)just outside the wedding hall and asked us to join him at the Sunday afternoon wedding. So in we went. We had been parked next to a Russian tour bus with vacationing workers from USSR. Several wanted to sit behind the wheel of our German Ford Escort, and we complied. Then one of the men wanted to trade one ruble for one dollar...the Bulgarian gentleman intervened and said the exchange was two rubles for one dollar at the time. That's when we were "invited"..we had a grand time of it, and afterwards he walked us to some of the main sights in town and suggested a hotel for us. All this was done in his broken English(a traveling sales engineer who had been to UK many times) and my high school German.

Hope you are still planning the Bulgarian trip. It will be memorable for you.

Stu

https://picasaweb.google.com/stuarttower/Bulgaria1980SAnd2005#

Hello Thea,
I did not understand what time of year you plan to do this trip and if the trip is still pending. I am from Bulgaria and honestly it is a lovely country. 17 days are good enough to see and feel a lot of places. Besides Plovdiv (particularly the old town of Plovdiv) and Veliko Tarnovo with the Tsarevets Castle which were suggested above, I would also highly recomment Rhodopa Mountains (in the south of the Country - lying on the borders with Greece and Turkey) - fantastic mountain scenery, nice rural atmosphere, nice people, real organic food and very, very cheap.

I would suggest rentint a car from Sofia and do day trips or trips with a night stay outside Sofia. Public transport is OK, but compared to most of the European countries is less than great.

Other places that i would highly recommend:
- Rila Monastery (hour and a half drive from Sofia). You can combine it with going further south after the Monastery and visit Melnik (close to the border with Greece).
- Koprivshtitsa - nice small villages with houses more than 100 years old. Can be visited when you drive from Sofia to Plovdiv.
- Triavna and Bozhenci - this is near Veliko Tarnovo. Nice old towns (Bozhenci is a very small village). I can provide links and additional information if you are still interested in doing this trip.

Hope that helps.