Idea was to go both Rila Monastery AND drive short into Pirin park: that's to much for a day trip from Sofia. So we stuck to the Rila Monastery. Day we visited the kitchen was in renovation and seems to take a bit of months more. The Monastery is really great! We arrived early, around 10AM, there was almost nobody, at 11AM the big groups arrived and we were done making photo's (unfortunately not in the church or museum, also not WITHOUT flash.). My personal opinion is that without flash it's OK to make photos as it does not damage anything.
After this, we drove a bit further (around 10 minutes) the road into the mountains, you find a sign of St. Ivan, there you can do a short 30 minute hike up the mountain to the cave he originally lived in, there is a small church devoted to him to. It's not (as far as I know) part of the WHS but for sure worth to visit.