I've done a few tours in Holland and Europe....Holland in early May can be wonderful if you like the flats and, of course, it would be tulip time (April-early May)
Check out the bike barge tours if you don't want distance...but if distance is what you like, try the north part of Holland in May...more countryside...less sand. The most popular tours do s. netherlands, but you may want to go off the beaten path.
Also, France/Belgium is nice in May. Weather could be an issue at any time so prepare for rain riding and escape plans to museums or alternate routes if nec.

If you decide to go in March, consider Portugal...Lisbon has some nice routes tho you can get the winds off the coast ...something to consider...but the weather is very nice in March.

And the s. of france can be warmer in March too.