Our (aborted) plan was to use a Dutch Flyer ticket to travel from Peterborough to Amsterdam using the overnight ferry (the ticket is valid from any Greater Anglia station to any NS station - Peterborough is our closest, and we'd pay the fare to there; the cabin is extra to the Dutch Flyer cost) - then use the Interrail pass from Amsterdam to Hannover.
The return journey was the OBB sleeper from Vienna - using an Interrail 'day' + sleeper berth surcharge - to Brussels where we'd overnight, and use our last Interrail day to travel back home (as our homebound Interrail day) via St Pancras.
It would have been possible to get off the sleeper train, mooch around, and then go straight on home by Eurostar, but it felt like we'd be completely knackered - and why not have a quick look at Brussels while we're there?
I'm pretty sure that the day ferry from Harwich counts as an Interrail single day journey to/from your home country, but that the overnight ferry doesn't.
We didn't have enough pass days to test the waters, and the Dutch Flyer ticket is cracking value IMO.