Serbia is beautiful, lots of road kill and memorials of killed drivers so watch out on roads. There's loads of trails and double track in both Serbia and Bulgaria. You could follow the (EV13) through Bulgaria. If you're planning on going from Serbia to Bulgaria following the Danube passing the Iron Gates make sure to avoid Romania
Really hated the Danube part. Wild dogs, empty stores, doesn't feel safe, boring landscape. Going through the mountain range north of the Danube (forgot the name) should be better though from what I've heard
Serbia is beautiful, lots of road kill and memorials of killed drivers so watch out on roads. There's loads of trails and double track in both Serbia and Bulgaria. You could follow the (EV13) through Bulgaria. If you're planning on going from Serbia to Bulgaria following the Danube passing the Iron Gates make sure to avoid Romania