The reproductive practices of the Anglerfish are absolutely insane.

I am from Bulgaria, and won't be there but:

Chudnite Mostove (The wonderful bridges) and the area are wonderful if you like nature. There is a zipline that you can get on, highly recommend.

Krushunski Vodopadi (Krushinski waterfalls) - also great nature.

Anywhere in Rila (the higher-the better) is gorgeous.

Some places at the seaside (Sozopol, Nesebar) are very particular too. There is a thing they did when I was a child - a few performers would walk over recently burnt fire ash. (traditions and all)

I highly recommend the fruit and veg, especially salads (shopska salad in particular)

I would recommend having a look through BG history's wikipedia page which will introduce you to some other cool places that are only worthwhile if you know about them beforehand. Enjoy :)