The easiest way will be a ferry across the Adriatic Sea: I believe there's Split - Ancona. Pass not valid though.

Otherwise it's a very large detour via Zagreb - Ljubljana - Trieste. Not possible in a day.

To get to Athens you'll have to take another ferry. From Ancona, Bari, Brindisi... to Patras and then a bus/train combination.

An Interrail pass won't be good value for this trip. Train tickets are too cheap and you won't take many trains in the end.

All train routes from Split to Athens are going to have to go back up North to Zagreb and Budapest (there is a sleeper train), then Bucharest. It’s a long way round!

I agree that the most logical route is across the Adriatic by ferry - from Split, Zadar or Dubrovnik to reach Italy. Then back across on another ferry to reach Corfu, Igoumentitsa or Patras in Greece.

This might help:

Italy to Greece via Bari https://www.seat61.com/Greece.htm#london-to-athens-by-train-and-ferry-via-bari

On that same page is also details of the rail route via Budapest & Bucharest. The other options - via Belgrade are not possible.

Hi everyone, for your information the SNAV ferry between Ancona and Split is part of the Pass’s benefit. Pass holders benefit from a 20% discount for that ferry. For more information, please visit our benefits portal