Amsterdam to Paris round-trip

The Thalys high-speed train provides the only direct service between the two cities. You can easily book tickets at http://www.thalys.com If you book early enough you can purchase advance non-refundable/non-changeable tickets for as low at 25 euros each way, and print your ticket out at home. When the website asks you where you live or where your are picking up your tickets, say "The Netherlands".

Thanks for that advice, that just cut my cost in half! Do you know if that train stops in Brussells and would I be able to get off for a few hours and re-board later?

Yes, Thalys trains stop in Brussels. However, since Thalys tickets are for a specific departure date and time and include a required seat reservation, you would have to buy separate tickets for each leg.

For travel between Amsterdam and Brussels you can choose either Thalys trains or IC trains. No reservations are possible on IC trains. Buy a ticket and jump on any train you wish. Unfortunately, your only choice for direct trains between Brussels and Paris is Thalys.

Thanks again. So, if I wanted to stop in Brussels and Paris being my departure city I would have to by a Thalys ticket from Paris to Brussels and then buy a seperate ticket from Brussels to Amsterdam on an IC
train. Can IC tickets be purchased in advance like the Thalys?

The tickets for IC trains between Brussels and Amsterdam can't be purchased in advance via the web. You simply buy it at the ticket window at the station on the day you're traveling. Fare is around 35 euros for second class.

If you don't want to lock yourself in to a departure time for Brussels-Amsterdam, then buy an IC train ticket (€36.60) when you get to Brussels. If you can commit to a departure time well in advance, then you might be able to get a Smoove fare of €19 on a Thalys train by booking on the Thalys site.