The nicest places to have lunch are on the main squares. That means you should head towards either the Hof or the Lieve Vrouw. Both are ringed with cafes and restaurants, the better ones being on the Hof.
To be honest though, I don't know which is actually open on Monday. The places I would most often recommend, De Pastinaeck and La Scala are certainly closed then. There will be places open, you'll be ok if you just wing it. In general the quality of restaurants/cafes in Amersfoort is excellent.
The boat trips in A'foort run Tue-Sun at 13.00, 14.00 and 15.00. I'd take a look when you are here though as the canal in Amersfoort is more like a moat rather than the canals you see pictured in Amsterdam. Personally i don't think it is that much of a trip. The shop which runs the trips is called Waterlijn and is on Krommestraat, just off the Hof.
You can't really walk round the canal ring, rather the old city walls. This meets the canal at times. It is very pleasant in some parts. Modern and uninteresting in others. When I have visitors I do take them straight into the center but I do that via the Koppelport - …wikipedia.org/wiki/File:Koppelpoort_Amersfo… - both it and the area immediatley around it are worth visiting.
Amersfoort has a beautiful, well preserved, medieval center. You will I believe like it.
Edited: 5:08 am, April 12, 2011