With a car you could visit some more WWI sites in Belgium or France. Calais is not that far. Or the Ardennes: war history (although I keep forgetting which world war...) and nice hills, caves, and adventure sports and/or gastronomy.
Or visit small towns in the Netherlands making your way back to Amsterdam (towns that have nothing to do with that war), for example beautiful fortress town Heusden, and picturesque little Oudewater, or another fortress town near Amsterdam called Naarden-Vesting. Just look op google images of the fortress towns to see what I mean.
As for Kampen, you could consider doing that by train and maybe renting a car from Utrecht or Schiphol airport. Depends on what you find convenient, and the cost. (Parking in central Kampen may cost money.)
Cost of train: www.9292.nl/en > there are various options, see the results on the left. If you choose one through Zwolle you can go and explore lovely Zwolle along the way as well.
Best chance of tulips after mid April but no guarantees... It's nature. And some tulip bulbs died in last weekends downpours and flooded land... But surely not all.
I hope this helps. :-)
Edited: 6:42 pm, October 14, 2013