You can easily get to the 7 Rila lakes by car.
The drive is about 1h 30 mins from Sofia city.
Click here to open Google Maps navigation
Pros:
+ indipendency
+ no other people on board
+ no fixed departure times
Cons:
- driving in Bulgaria can be challenging as signs are often in cyrilic alphabet only
- parking near the Rila lakes charlift is difficult during summer season and weekends
- parking fees apply