I can only recommend this tour as you get to see two of Bulgaria's most popular attractions in a single day.
Our guide and driver Petar was very knowledgeable and he remained enthusiastic and friendly throughout the tour which is quite long (12 h).
I suggest bringing a warm jacket and adequate hiking shoes. Also bear in mind that there is quite a bit of driving (about 4 hours in total) but the views are great most of the time. Cash is needed for the chairlift (20lev per person).