Lee, I don't mind doing part by bus. It will be a bit of a distance for a day trip, but I think it will be worth it. I checked on DB and it looks like it would be faster to take the train to Mühlacker and then a 20 minute bus to the monastery, rather than taking the train to Maulbronn.
Karen, thanks for the description! It looks really lovely and it sounds like it would definitely be worth a day trip there. I wish I had time to spend a few days in the village but it would mean eliminating Cologne and Aachen and I hate to do that. But a day trip there will be nice and I can always come back another trip.
Nigel, there are a number of hikes around there as well, and a beautiful nature park. By the way, the friend who recommended it stayed at the Hotel Klosterpost in Maulbronn and said it was very nice. Unfortunately, I only have time for a day trip but it would be nice to come back and stay longer.