Viking includes some excursions in each port, so those will fill much of your time in the days not in Amsterdam. In Amsterdam - with several days - you can both take advantage of the Viking tours, but add to your own list of "must sees" - perhaps gleaned from RS guide books.
It looks like an Amsterdam canal cruise and a trip to Keukenhof are included excursions, so it just makes sense to take advantage of them. Having done both, they are very nice to do. Amsterdam is deep in history and art options, with the obvious ones being obvious because they are genuinely very good. The AF house and the Resistance Museum are a great pair (and add in Haarlem's Ten Boom house). The assorted art museums like Rijksmuseum and the other museums clustered around the Rijks are an easy win.
We enjoy wandering cities, and Amsterdam is one of those that is quite nice (and easy) to wander. Find restaurants and bars to relax and have tasty food and drinks. There a MANY good food and drink options. RS has a couple walks that are easy, but those would just be "starting" points for several days of exploration.
Personally, my wife and would immediately be renting/borrowing bikes and getting around that way. Amsterdam has fun cycling options in all directions from the downtown, and getting rapidly out to the slightly less dense areas is easy. If the weather is agreeable, I'd dedicate at least a day to riding outside the city's heart.
One fun and relatively easy ride we did from Amsterdam in May was a recommendation from RS for the Wetland Safari canoe adventure. We rode out from the city, had a lovely time canoeing with the guide/owner, and then she gave us a great suggestion for a longer but very fun ride back. With longer days in early May than in April, we had plenty of sunlight to take advantage of.
If you haven't already, maybe look at the timing of in port & leaving port. That will give you a better sense of how much extra time you have once you have also taken the included Viking excursions.
Also, pop over to Cruise Critic and start looking through the Viking posts and the Roll Call for your cruise plus past cruise recaps.
