Founded in the 10th century, the Rila Monastery is the most famous Eastern Orthodox monastery in Bulgaria and regarded as one of Bulgaria's most important cultural and historical monuments. The visit to the monastery and the main church is for free, but I am suggesting not to miss the main museum (costs 8 leva). Also, I recommend taking a walk outside the monastery, where you can find Small booths with souvenirs, restaurants, and the monastery bakery, where they will serve you a Mekitsa - Bulgarian donut with a lot of sugar powder.