Buy the KPN prepaid SIM card (available at any cell phone shop), charge it with 10 Euro, activate your data plan by calling *132# (star 132 pound key)
Data plan will cost ya 9,95 Euro and you can use 200 MB for that within 1 month.
Be careful, any MB over 200 will be charged at 10 Euro per MB
When you leave NL dial *132*009# to de-activate the data plan (or just toss the SIM card)
If you find all this too difficult, just ask the shop assistant to help ya out.
