"4 countries, 6 days" Top 5 Page for this destination Amsterdam Transportation Tip by ATLC

For a holiday, I agree that 6 days is a very tight schedule. Not really my idea of a holiday. But I've seen much worse on this forum.
However, if you were on business, you would not blink at this schedule.
We have the fast train Thalys after all.

Let me suggest an itinerary:

Day 1: (on arrival): take the Thalys to Paris (you need to book Thalys in advance), travel = about 3 hours.
Day 2: Spend in Paris
Day 3: (in the morning): take the Thalys back to Brussels (Belgium), travel = about 90 minutes
Day 4: take a train to Aachen (Germany), which is just on the Dutch/Belgian border. Enjoy the cathedral, etc. Travel time: with Thalys: 70 minutes. With local train: 2 hrs and 30 mins
Day 5: visit Maastricht (20 kms from Aachen, very nice Dutch city) and/or do that 'Drielandenpunt' (Belgium/Germany/Netherlands) as johngayton suggests
Day 6: back to Amsterdam and enjoy any time left there. Train from Maastricht to Amsterdam (local, just hop on). Travel time: 2 hrs and 30 mins

For German and European train schedules: http://reiseauskunft.bahn.de/bin/query.exe/en
For Dutch train schedules and prices: http://www.ns.nl/cs/Satellite/travellers
For Belgian train schedules and prices: www.b-rail.be/main/E/
For Thalys: http://www.thalys.com/ (only to be booked in advance!)

Mode: AROUND
Type: Train