Balkan (Балкан, Стара планина) - This is the mountain that separates Bulgaria's north from the south. The mountain stretches from mount Kom near Bulgaria's border with Serbia to the cape of Emine at Black Sea. Near to its highest mount Botev is located the 124.5m waterfall of Raiskoto praskalo (Paradise's shower).
Rila (Рила) - home of some of the most beautiful monasteries. The most famous one and a must see is the Rila Monastery. Rila is also the highest mountain in the Balkan peninsula with highest peak mount Musala (2925 m). A very beautiful site to visit is the region of the 7 Rila lakes, which is considered holy to the White Brotherhood and attracts pilgrims in Summer and on their "Days of the New Year" in August.
Pirin is located in the Southwest part of Bulgaria. While not very big in area, it is the second highest (Vihren peak, 2914m) mountain in Bulgaria. Often considered as the most beautiful mountain in the country, it features beautiful landscapes and many lakes. There are several touristic huts. Very good for hiking in the summer (July and August).
Rhodope Mountains occupy a big part of Southern Bulgaria. While it is not very high, it presents a challenge to the travelers due to the limited number of roads and the steep hills and the deep forests. The people living in the Rhodopes are very interesting to communicate. Pamporovo (Пампорово) near the town of Smolyan in the middle of the Rhodopes is one of the most famous winter resorts in south-eastern Europe.
You will want to stay in Sapareva Banya, the main town and one of the main launching points for hiking Rila Mountains and the Seven Lakes. You can also stay directly in Panichishte (a "town" about 5km from the base of the chairlift) or in a cabin at the top of the chairlift. Just keep in mind that doing so is unnecessary. See "by public transport".