Where are you stopping? If Krems then a bike ride seems like the call. Not at all a bad town, but the bike ride to and from it is sublime. Easy ride up stream on the Krems side of the Danube for 10 miles and back.
But if Melk be aware that to get into the Wachau valley you'll climb a bit. And if you want the north bank (which most do) you'll cross a safe but large and somewhat time consuming freeway bridge. If you were going to ride 15 miles on the north side of the Danube into the Wachau from Melk, 2/3s of it would be a steep hill and big highway bridge coming and going.
Plus you'd miss the Melk Abbey, certainly one of the greatest Baroque sites on the planet, probably the most mind-blowing "church" on most Danube cruises.