If we have to rank the hiking and natural spots in Bulgaria according to how famous they are, Seven Rila Lakes in Rila mountain will undoubtedly get the first prize. And of course, there is a solid reason why the seven lakes attract so many tourists per year - the trail is suitable for beginners, and the beauty of nature as well as the amazing views you get to see are breath-taking and will definitely make you want to stay in Bulgaria longer and explore more and more!
I personally have visited this place twice - one time being over the weekend and one time on a weekday. I highly recommend planning a trip to Seven Rila Lakes on a weekday when there are less tourists - over the weekend it can get quite overcrowded (and unpleasant). Both times I travelled by car from Sofia to Panichishte (a winter ski resort that is the nearest to the lift that takes you to Rila Lakes hut - the starting point of the trail) and I do believe this is the most convenient way to reach this place. It takes around one hour and a half from Sofia to Panichishte by car. Another option is to go to the town of Dupnitsa (around one hour away from Sofia, you can reach it by bus or train) and there you can catch a bus that takes you to Pionerska hut (close to the lift) or even straight to the lift cashier desk. However, you should call in advance the bus station at Dupnitsa and ask about the bus schedule. After Dupnitsa you head to Sapareva banya (a town located at the foot of Rila Mountains and famous for the hot mineral springs) and then the next destination is Panichishte. The road between Sapareva banya and Panichishte is quite scenic as you go up the mountains and the view over the surrounding region is really nice. If you are coming from Plovdiv, you won't visit Dupnitsa, but you will pass Sapareva banya too - travelling by car from Plovdiv to Panichishte takes around two hours and a half.
It is possible to visit the Seven Rila Lakes for a day and go back to Sofia (or wherever you are coming from), but I think it is a good idea to spend the night in Panichishte. Close to the winter resort there are other hiking trails (for instance, to Rilska Skakavitsa waterfall) that are really pleasant and you should definitely visit. There are plenty of hotels in Panichishte and the resort, in my opinion, is quite children-friendly.
In order to go to the lift that will take you to Rila Lakes hut ("hizha Rilski ezera") where the trail for all seven lakes start you have to travel a bit more though. Around 5 km away from Panichishte you will find the lift as well as a parking lot next to it. You can leave your car there and catch the open lift to Rila Lakes hut (here is more information about the fee and the opening hours of the lift). The "open lift experience" takes around 30 minutes and it is very scenic - if you turn around you will see the area around Rila mountains and you will definitely take a lot of photos (in case you are not scared of holding your phone while being at an open lift...). You also have two other options: one is to go by foot from the parking lot (or even from Panichishte) to Rila Lkes Hut - there are white-red-white signs that will take you there. I think this is a nice opportunity, but it will make your hike longer (maybe add an hour or two). I do not recommend the third option - there are people who offer "off road exprience" to take you to Rila Lakes hut. Many ecologists say that these jeeps really harm nature. In my case, one driver even lied to me that there are queues at the lift, so he claimed it was better to use his services instead. I didn't believe him and I went to the lift ... and there were barely any people waiting there! Indeed, the lift experience is quite pleasant, but if you are afraid of heights or of open lifts in general, better hike from the lift to Rila Lakes hut.
Once you reach Rila Lakes hut (it is a big hut, you won't miss it) you have two options. The first option is to go right follow the route marked in red. The first 15 minutes are a bit steep, but then once you reach the hill it is pretty much a pleasant walk with nice views until you reach the Babreka lake ("the Kidney lake" - because its shape looks like a kidney). The other option is to go left and follow the yellow sign direction Seven Lakes hut ("hizha Sedemte ezera") - this way you will pass pretty much all lakes (you will see the following lakes: Dolnoto ezero, Ribnoto ezero, Trilistnika, Bliznaka and at the end you will reach Babreka lake and the red route). I know the second route might sound better because you see a lot more, but I personally didn't like it - the first one is more scenic, shorter and pleasant. The second one is covered in stones (so your feel will suffer) and much longer. Also, you will see all lakes from above (once you reach the last lake), so you won't miss much if you take the red route, in my opinion. If you choose the red route, you should reach the Babreka lake in hour and you can have lunch close to the lake.
After visiting Babreka lake you go up the mountain and follow the yellow or the green signs - they will both take you to the "Lake Summit" ("Ezeren vrah" in Bulgarian language). The Lake Summit is the place where you will enjoy a view over all seven lakes. After visiting Babreka lake and hiking for about 15 minutes on a steep trail, you will reach Okoto lake ("The eye") - even in summer you should be able to see some snow there which always fascinates me. There the route goes left to reach the final lake - Salzata ("The tear"). Before you go left, I recommend you hike for a few minutes on your right side and follow the blue signs direction Kabul peak and Otovishki peak. In a few minutes you will also end up at a spot with a really nice view over the lakes around you (and it is less crowded). After you take photos there, simply go back to Okoto lake and the green/yellow route. In another 15 minutes hike on a steep trail you should reach the Lake summit and the last lake. This is where most people take photos of all lakes, but here I can also give you one more tip - continue walking for 10 more minutes on the same route (green/yellow) until you reach the next hill. There are less people and the view is even better, in my opinion. This is where I took the photo above.
In total it took me around 2 hours to reach the Lake summit. From there you can simply go back to the lift (it should take you around 2 more hours). I personally continued the hike along the green route which takes you to Razdela area, Pazardere area and then to Ivan Vazov hut. From Ivan Vazov hut there is also a 7-hour trail to Rila monastery, which I definitely have on my to-do list for this summer. In any case, if you are into hiking, I recommend getting a map of Rila Mountain - you will see that there are many trails around the lakes, so you can combine your trip and visit other huts nearby.