Some great locations have been suggested here for "showing off" Bulgaria. I'd like to suggest another... About two hours away from Sofia is a beautiful village called Koprivshtitsa that will really give them a taste of old-world Bulgarian culture.

We stopped there for a few hours last year on our way across to the Black Sea and it was truly amazing! Some of the most beautiful pictures I brought home from Bulgaria came from Koprivshtitsa.

Anyhow, good luck "selling" Sofia, and the rest of the country. Don't forget... ultimately, its about the people!