Hello from a Bulgarian-based travel lover!
Every time I travel from Sofia to Thessaloniki and back I'm amazed at the hills and mountains seen from the low-lying planes in the area of Greece between the border at Kulata-Promachonas pass and Thessaloniki.
The valleys in Greece are lower than ours in Bulgaria and, while the mountains in the area aren't taller than ours the difference in relief is more imposing than anywhere in Bulgaria. We don't have this huge contrast between the altitude of valleys and mountains or the valleys are so tight you can barely enjoy the difference in scale. The change of elevation here is generally more gradual. Our lowest-lying valleys are next to the Black Sea and the Danube but they're far smaller and usually away from mountains and even large hills.
I urge you to visit this little-known area of Greece.
Photos and Street View don't do the area justice, you have to drive or take the coach from Sofia to Thessaloniki (or Serres, or Kavala) to really experience it.