Trains connect Zagreb to Italy, Germany, Austria, Hungary and Romania, while ferries link Croatia to Greece and Italy. Around Croatia Airlines has daily flights from Zagreb to Dubrovnik, Pula, Split and Skopje. The country's excellent bus network is far-reaching and reasonably priced.
Trains are cheaper and more comfortable than the bus, but also a lot slower.
All the usual car rental chains are represented in Zagreb and Split. Drive on the right side of the road. The spectacular Adriatic highway from Italy to Albania runs along the steep slopes of Croatia's coastal range, and is one of the most exciting drives in the world. If you can get where you're going by ferry, then do it - cruising among the islands of the Adriatic certainly beats sitting on a bus, no matter how cheap or fast it is. Jadrolinja ferries ply the waters between Dubrovnik and Rijeka.