Belgrade is nothing spectacular. Safe? es no problem there, the usual big city problems of course. Belgrade isn't a pretty city, it's gritty and there just isn't the sights that attract the average traveler. Also, you'd be a full days train ride from anything else of interest. Unless you are really into Serbian culture, you'd be best to focus your time elsewhere. Bosnia is a new and upcoming traveler destination and well worth your time in Mostar and Sarajevo. Croatia of course is amazing, Slovenia as well. If you haven't been to Romania or Bulgaria, you'll find them fascinating places. Ukraine has recently dropped visa requirements, so your passport will open you up to a great city called Lviv, a train away from Krakow, Poland.
Hope that helps,