[Marco]

Have some business there and will be around half of the time, in the other half I consider a side trip, will have a car.
Bucharest is an option but it is a bit far to drive, also considered Nis, Serbia. Or maybe 2nd tier in Bulgaria (?) but not the beach resorts.

[chexmix]

skyscanner.com

It will open your options of where you can go much more and many round trip flights can be found for very cheap. I'd consider hitting up a coastal area, Croatia was pretty good just a few weeks ago for me and all the land locked cities I visited, paled in comparison.

[K-man]

Just stay in Sofia, people will be already back from the beach at that time.
However, Friday Sep 6 is public holiday in Bulgaria so the long weekend the city may be kind of empty, with people making a last trip to the beach, driving down to Greece, or visiting their country houses to pick grapes and apples.
Anything within 2 hours drive of Sofia will be worse in quality and quantity of girls.
Nis is not worth it. If you want to go to Serbia, go to Belgrade.