4 day trip to Bulgaria. Tips plz :P

[IrishBlood]

Some friends and I will be traveling through Bulgaria from around the 13th to the 17th this month. We will be going from Thessaloniki in Greece to Sofia, then Plodiv, then Veliko Tarnovo, Ruse and then on to Romania.

Is there anything we should try and do along this route? I think we picked good cities/towns that would give us a good idea of what Bulgaria is like (as opposed to just seeing Sofia). Are there any must dos I should know about?

[Teutonic]

Keep an eye on the weather and its potential effect on the road conditions. Obviously the tricky sections, if driving, are between the Greek border and Blagoevgrad and crossing the mountains on route to V.Tarnovo from Plovdiv.