Melk Abbey is a great visit and easily reached via train.
depending on the weather and time of your visit , I would not discount sometime along the Danibe - by bike or other means.
A locsl bus runs almost every hourn beteween Melkj and Krems
some interseting sights there.
then train back from Krems to Vienna.
several choices for bike rentals too- if you visit after March 20 - the Leihradl/ next bike system will be operating-
pick up bike and drop off where you wish . many kiosks in convenient locations for this.
1 euro per hour 5 euro per day
or a rental near to Krems in Mautern - just across the Danube from Krems.
If biking from Melk toward Krems - any wind is usually at your back .
Good bike pathways - great scenery.
March-probably many vintners working in their vineyards - for sure a place to stop for some wine and food too.