Favorite thing: On the way from the Wachau region back to Vienna, Michael (VT member globetrott) drove my wife and I up the the hill to visit the famous Stift Gottweig (Gottweig Abbey). When we reached the abbey, it was sunset time and we managed to witness the beautiful sunset at the Wachau region of Austria. It was a romantic moment for me and my wife :)
Personally, I think summer would be a good season to visit Krems and Wachau area because of the warmer weather, flowers, greenery and also to make use of the longer daylight hours because of the travelling distances within Wachau as well as from Vienna.
Also, most of the attractions will be opened during summer. In order to minimise the crowds, it would be better to visit during early summer May-June as compared to the heart of summer (July to August).
There is a big and impressive map of the old town area of Krems before you enter it. It marks out all the famous spots within the old town area.
If you want to explore the entire old town of Krems, it will take several hours but it is worth it because of the beauty of the town's architecture.