When you visit Eastern Europe you can mostly choose from three ways of transportation: Car, Bus or Train!
Rent a Car
The most flexible and convenient travel alternative but also the most expensive one is to rent a car. You can find many companies at airports. If you are in Bulgaria a good company, which also speaks English, is Sofia Auto. Prices are ok and their service is good. Remember that you need to be 21 to rent a car and you need an international drivers license if you are non-European.
If you travel from a European country to a non-European country like Macedonia, make sure you ask for a European green card. It’s also called a vehicle green card, which you need in order to be insured in those countries. Before you start your travels make sure if you need one.
In case you forget to ask for one, there are insurance bureaus at most borders and they sell you a 15 days insurance for about 50 €.
Go by Bus
If you like to travel cheap and be relatively quick and flexible you might want to consider taking a bus. Buses operate often between cities and every big city has a national and international bus station that connects you to other countries.
Here are some links of the websites of international bus stations in Eastern Europe:
Sofia Bus Station: http://www.centralnaavtogara.bg/index.php
Budapest Bus Station: http://www.volanbusz.hu/
Skopje Bus Station: http://www.sas.com.mk/
Podgorico Bus Station: http://www.autobusni-kolodvor.com/en/
Sarajevo Bus Station: http://www.centrotrans.com/loc/
Bucharest Information: http://www.romaniatourism.com/bucharest.html
Go by Train
The railway network in Eastern Europe is not very good and not developed and the trains, most of the times, are old and dirty but it is cheaper to travel by train than to travel by bus. You should have a lot of time though. Trains usually stop at every single stop and for a trip that takes 1,5 hours by bus you will need at least 3 hours by train.
To check timetables you can use the website of Deutsche Bahn which has timetables for most trains in Europe!