The train costs in total (back and forth) for 2 persons between 147 EUR and 167 EUR.
Which means a total per person of about 75 - 85 EUR.
The trip would take about 3hrs and 30 mins. each way.
Furthermore you have to switch trains in Aachen as well as Liege.

This is just an example calculation if you would leave Krefeld on the 23.8. and come back a day later.

You sould also consider taking a trip to Berlin! (I love Berlin)
And since you are close: Cologne, of course!

Have a great trip and fun!

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Yahoo! 4ever