We did a tour of belgium – Hull to zeebrugge ferry overnight, then visited Ypres, Ghent, and Bruge. That ferry no longer runs but you could do something from Rotterdam
Go to Oban and get the ferry to the Outer Hebrides. Ride north, then either ride back, get a bike transfer back, or get the ferry over to skye and ride back down the mainland (or to Ullapool and get a train)
Get a ferry from Anglesey to Dublin, ride down to wexford, ferry to FIshguard and ride back up Cardigan Bay
Get up to Whitehaven and ride Coast-to-Coast over to Tynemouth. If you;ve time, rideback along the Reivers route