Is it possible to get a direct train from Belgrade to Split or do you have travel via Zagreb? It is possible that Dubrovnik could be an alternative for Split, so if it isn't possible to get directly from Belgrade to Split can you get directly to Dubrovnik from Belgrade?
I'll be in this area during July (about the 24th/25th) if that makes any difference
Also I was wondering if it would be better to travel to Ljubljana or Belgrade? My friend and I can't decide... is either noticeably better than the other, or are they both worth a visit?
You can go directly Beograd - Zagreb part, not via Budapest. There are 4 trains Beograd - Zagreb, at 5:45, 10:20, 15:25 and 21:40.
Few last summers, there was once weekly direct train Belgrade - Split, which departs Belgrade at arround 2 or 3 PM, every Friday and arrives next morning in Split. But, every year, it is announced just week or so before it starts running in late June or early July. So, it will most probably run again this summer.