If you know exact dates, you could also book Paris-Brussels (ZYR) return, and Brussels-Amsterdam return. If you book as early as possible a Thalys ticket may well be cheaper than a normal train ticket. If you go to the Netherlands for the weekend, you could save (on the normal train) by booking a "Benelux Weekend" ticket. Also, return tickets during the weekend are almost 50% off in Belgium.
Reservations open 3 months in advance I believe ; internet reservations possible ; ticket pickup at a distributor. It's probably advisable to do this in advance, I once saw a guy having to cough up like 500 Euro for his ticket because he had some trouble with the distributors (his claim, he showed a reservation printout to the conductor),
Thalys tickets when booked at last moment tend to be horribly expensive. It may be worth looking at 1st class tickets ; I recently booked a 1st class ticket ZYR-CDG that was cheaper than anything left available in 2nd class (cheaper fare bucket). The most economical Thalys tickets (Smileys) are return tickets, usually sold out VERY quickly.
Thalys is a company owned by several European railway companies ; they run trains Paris-Brussels-Amsterdam, and Paris-Brussels-Cologne (they used to run to CDG as well, this line is now run exclusively by TGV (high-speed subsidiary of SNCF, the French railway company).
Bus : Eurolines as mentioned before. Cheaper and more flexible than Thalys, but takes 5 hours Paris to Brussels, only 2 or 3 departures per day, v.s. 1hr25, 20 departures per day or so. Thalys Brussels-Amsterdam is little faster than the traditional train.