Or you could do Melk and Durnstein as a day trip from Vienna.

Great day trip from Vienna - take the train to Melk - less than hour and visit the Uber famous Melk Abbey and stroll down to the boat dock and hop the frequent enough boats in season to Durnstein - then take a train from there back to Vienna or continue by boat to Krems, a larger but still sweet Danube town also with train service to Vienna (different route to a different station than the one to Melk)

Those boats thru the Wachau Valley - probably the prettiest stretch of the whole Danube from Black Sea to German hinterlands - present many possibilities - if you want to do a little biking in one of Europe's primo biking venues - on bike paths along the Danube thru the vineyard-carpeted and castle-studded Wachau Valley then the boats provide them and you can pedal between boat docks.