Bulgaria is one of the cheapest countries in the EU to travel in, and these two cities are a good pair: Sofia is a working capital with a mountain on its doorstep, Plovdiv a small, old and very pleasant place to slow down.
Day 1 — Sofia's layers
Start at the Alexander Nevsky Cathedral, then walk to the Serdica complex, where the ruins of the Roman city are exposed around and inside the metro station. Nearby are the 4th-century Rotunda of St George and the Banya Bashi mosque. Fill a bottle at the free mineral water taps by the Central Mineral Baths — locals queue there with jerry cans.
Day 2 — Vitosha
Take bus 66 from Vitosha metro station up to Aleko, at about 1,800 metres on Vitosha, the mountain that rises to almost 2,300 metres right behind the city, and hike to the summit, Cherni Vrah, or across the plateau. The lifts up from the city are out of service. On the way back, stop at Boyana Church, with 13th-century frescoes on the UNESCO list; visits are short and by small groups.
Day 3 — Rila Monastery
Spend the day at Rila Monastery, about two hours south of Sofia by road. There is no direct public bus any more (the monastery suggests a bus to Blagoevgrad and a taxi from there), so most people take a small-group tour or share a car. The painted arcades of the main church are the image of Bulgaria you will have seen everywhere.
Days 4–5 — Plovdiv
Take the bus (around two hours) or the train (closer to three) to Plovdiv. Walk the cobbled Old Town, with its 19th-century Revival-era merchant houses, to the Roman theatre, which still hosts concerts in summer. The Roman stadium lies partly under the main pedestrian street.
Eat and drink in Kapana, the old crafts quarter, revived around the time Plovdiv was European Capital of Culture in 2019. On the second day, climb one of the city's hills (tepeta) for sunset.
Getting out
Plovdiv's airport has few flights, so most people return to Sofia to fly — or continue to Istanbul by bus or train.