Have a look at seat61.com for possible routes. Or take a map, look for cities you'd like to visit (there are loads of them that are worth a visit) and then use a planner (e.g. the DB planner) to check timetables.

There are quite some ferries from Italy to Greece. Do a Google search or look here for the ones that offer a discount with Interrail:

https://benefitsportal.eurail.com/?categories=1be1ba1c-80f5-4286-b255-3092f0d486cd

When to book trains very much depends on which route and when you'll travel (date & time). Book any night train as soon as you know when you'll use them, especially for the summer.

Thanks so much! Very valuable advice and links. Of course we did some googling but we wondering if anyone had some nice tips on the route to take. 

We are now considering:
Utrecht - Vienna - Bologna - Ancona - Greece 

or

Utrecht - Vienna - Venice - Greeece. 

We are using the interrail app for scheduling and booking night trains, but this does not seem to work yet (see the image attached)

Hello,

For planning your itinerary it is generally not recommended to look at the Eurail Rail Planner, it's not up to date.

For this leg you should search for the train company running this train, in this case ÖBB/Nightjet.

On their website, you'll see that the night train is already available (usually for other trains it's 60-90 days before).

340€ for a private couchette carriage though… It's cheaper for other dates so please have a look.

Thanks so much! I managed to book it via the OBB website. It feels sort of strange that the Interrail app is not up to date… One more question, if you want to book trains from Vienna to for instance Ancona you have to book it also via the Italian railway? 

Thanks!

It depends on who operates the train.

There is a Nightjet between Vienna and Ancona during summer. ÖBB are a big player in the night trains world !

Departure at 19:18 and arrival at 09:00.

But this route could also be done during the day. For example Vienna - Venezia Mestre - Ancona with a departure at 06:18. It would be a very long travel day so if you choose the day trains, maybe have a stop in Venice too.

Italian trains should be booked on ÖBB too, don't ask me why…

This guide will help you : 

There are no pass holder prices for a private couchette, only normal fares. If you're with 4 people, just book a 4-person couchette via tickets.oebb.at and it will be €172. You'll be placed in the same compartment if possible and if it's not possible you’ll see a warning before you pay.

Note that if children are under 6 years but you want them to have their own berth, then make them 6 years old. If you book couchettes (or a sleeper) via ÖBB for children under 6, then they won't have their own berth.

If you visit Bologna check out the Mercedes B&B, small clean and 5 minutes walk from the station. Very popular but as good as any - basically a small apartment in a block of modern shops and offices.

If you stop in Venice then look at hotels in Venice Mestre  - cheaper and better than Venice centre,

Thanks for the info ! I hadn't thought of the warning and it's really useful (also for my incoming trip maybe...).

Thanks so much, very valuable. I think we’re in the same couchette, because I did not see a warning ;) 

Thanks, I am going to check it out. Especially Venice is really expensive indeed, so I will look at Mestre.