Boyana Village and Church, Sofia
This little church from the 13th century in on Unesco's Heritage list. It's interior is decorated with beautiful murals which date bach as far as 1259. You'll only be allowed into the church with a guide who is definitely quite informative.
Admission for foreigners is 10 Leva.
Opening times: 9am-5pm, Tuesday-Sunday
Minibus No.21 (thanks to VTer Boyan18 for this info) is going from the centre of Sofia to Boyana.
The Boyana Church is on the UNESCO World Heritage List.
The church is interesting because of its architecture and frescoes.
built in three stages: 10th/11th century AD, middle of 13th century and middle of 19th century with several layers of frescoes: the first one dates back to 11th/12th century and the second one - to 1259 and late frescoes from 14th, 16/17th and 19th century.
Follow this link to see some pics from inside.
Admission fee: 10lv
There is a combined ticket for National museum of history and Boyana Church - 12lv.
Boyana is situated in the outskirts of Vitosha and is one of the best quarters of Sofia. There you'll find the small Church of Boyana which is a "world masterpiece" and is included in the Unesco list of the world cultural heritage. The frescoes were made by an unknown artist in 13c.
From the center of Boyana you can reach the Boyana waterfall (1h-1.5h climbing) and look at Sofia from above.
Boyana Church - on the UNESCO list of world treasures, this church was recently restored and more beautiful than any of the pictures you would ever see of it.
You can take a taxi from Sofia top the Boyana suburb, which costs about 3 leva ($1.50) and it takes about 10 minutes. On the way back you can take the 'route taxi' #21(in Bulgarian MARSHRUTNO TAXI), which will take you back to the center on the city at NDK.
You can see a nice photo of the interior of the church and read more historic information at this site http://www.sofiaecho.com/daytrips.php
There are other fun day trips mentioned on this site, but I did not go to any of those places.