Just seen this thread.
Gouda has a market square, a town hall, a cheese museum and that's it. Half an hour tops. Try pushing on if possible.
The dyke from Enkhuizen to Lelystad is definitely doable. There is a cycle path alongside the road. I prefer this dyke to the Afsluitdijk (quieter, got a curve in it, more to see and only slightly shorter). Be aware though that cycling along a dyke might not be as great an experience as you might think. You will always have the dyke (i.e. a grass embankment and no view) on one side of you. The Markerwaarddijk (which turns into the Houtribdijk halfway across the Ijsselmeer) has the additional disadvantage of ending in Lelystad (an ugly town built for cars and a bugger to find your way out of) which is in Flevoland province (deadly dull but can fortunately be exited in a couple of hours).
Of your route suggestions, Groningen is a nice town but the countryside around it turns a young person's thoughts to suicide. If you go over the Afsluitduijk the eastern side of the Ijsselmeer is very nice and will give you access to the Veluwe (definitely worth visiting) although a grand tour with six novices in one week may be asking a bit much.