You can use the bus which goes about every hour, some go into Sunny Beach not all and the others go to Burgas. Price is about 38lv per person one way. The journey takes about 6 hours the bus stops halfway at Stara Zagora so you can go to the toilet and get something to eat, if you are going to use the bus you wil have to book from Sunny Beach for the return as they get very busy from there.
You can get the train which takes about 7 hours very slow and not that clean on the trains toilets are a no no. This will take you to Burgas so you will have to get from Burgas to Sunny Beach, there is a new sleeper train from Sofia to Burgas I have not used it but some friends of our did this winter up to Sofia they said it was quite good and its a new train.
You can get the internal flight [url=http:
//www.
bulgaria-air.
co.
uk:3vo4mlye]
[You must be registered and logged in to see this link.] this goes everyday from Sofia, takes about 40mins on the plane for June the price is showing 56lv the flight is not until the evening so you migh have a little time at Sofia but you could always get a traxi into Sofia to have a look around, if you you use the internal flight and you are coming in on a Wizz or Easy Jet flight you will come into terminal 1 and the internal flight goes from terminal 2. The internal flight arrives about 9pm ino Burgas, you could then book an airport transfer, which our company offers. Hope this has helped a little