oeps, you were talking about may in stead of march,
here is another shortlist:
roadraces:
The Hague royal ten / The Hague / 10 miles / 21-5
Run2dayrun / Utrecht / 10km / 21-5
Marikenloop / Nijmegen / 5-10km / 21-5 / women only
track:
Nijmegen Global Athletics / Nijmegen / 24-5
nice little comp where they mostly run 800m - 1500m and a 3 or 5km. Kenny ran there a few years back and you can see some nice races there (1.46 / 3.37 /half or low 13's)
Hengelo FBK Games, IAAF Grand Prix / Hengelo / 28-5
if you get a chance, would be nice to see the big boys at work! the competition is almost known as a runners event only although they have a few technical events. I believe last year they ran a 10k, 5k, 3k, 800m and a steeple chase for men, with a 5k and 1500m for women. I am sure either Kenny or Sileshi will attack the WR over 10k so it is worth missing your flight for