During my backpacking journey in May, I will be heading from Athens to Brasov. There's a whole lot in between and I don't particularly want to spend that much time consecutively on trains. I have about 3-4 days in between. I'm thinking of hitting Plovdiv in Bulgaria. However, the Athens - Plovdiv and Plovdiv - Brasov journeys are both fairly substantial.
Any suggestions on rad things to do in between?