I've been following your blog for awhile now because I applied for the ETA in Bulgaria for this coming year...it was my way of both living vicariously through your experience if I didn't get it, and to kind of mentally prepare if I did. And it's done the trick!

Anyway, I visited Bulgaria a few years ago to visit some friends there and, after Sofia, they took me to Plovdiv, Rila, Bansko and Varna (that's obviously too far). Since you've already been to Plovdiv and Rila (both of which I love), I highly recommend Bansko. Even in the spring/summer. For me it provided a really stark/wonderful contrast to Sofia, and expanded how I saw Bulgaria, with the little restaurants with the traditional pottery/tablecloths, etc, and the wonderful babas EVERYwhere, and more of a folk culture than I remember in Sofia. There was more to it than that, but it's what I remember off the top of my head. At least look it up!

And, in your defense, my memory of Sofia is wonderful. I don't remember it being all that dirty (though I'm from Philly?), and I'd live there any day. Have fun on your trip!