I drove from Sofia because public transport (a bus which doesn’t run regularly and will only take you to nearby Bansko) seemed a bit complicated and I was tight for time. I parked somewhere near the Vihren hut and hiked up from the hut for about 3-4 hours. In hindsight, and of course if I had more time, staying in Bansko would have been a decent option as it looked like a nice albeit probably quite touristy place.
There are colour coded maps all over the park. I seem to remember taking the red coloured path from the hut and came across a lot of the big lakes. Quite a few people (I think a decent amount were locals too) seem to use these areas by the lakes as picnic spots, and even more people were going on long hikes, which was quite surprising as I expected it to be pretty quiet. But it seems that lots of people go to this park!
I was really surprised how beautiful this place is given the somewhat low average review. The lakes are really the highlight and make the sometimes strenuous hiking really worth the while. The view of the surrounding mountains, especially the big mountain Vihren, is also pretty cool. Perhaps it is just a lot nicer during the summer than the winter when the ski resort is in full swing. Who knows, but I really liked this place. Didn’t see any brown bears or snakes though, which I’m told is a thing there. Try the traditional white bean soup from the Vihren hut restaurant, which I was told has herbs and mushrooms from the surrounding area. And of course try the water from some of the streams in the mountains where other hikers refill their bottles.