If I buy a Ticket from Thalys Amsterdam–Paris, do I have the option of stopping over in Antwerpen, or maybe Brussels just to walk around and then take the next train?
How expensive will it be?
When I took the Thalys just to Belgium it did cost me the same as when I took that train to Paris, or more as I did not managed to get the lowest price for the tickets. From that point of view, you will pay an extra leg of travel, likely for the Belgium to Paris leg it is easier to find cheap tickets as there are more trains.
But all depends on when you buy the tickets.
Best search on a website that shows the prices for the tickets for the timespan from now when you will buy your tickets, if you want to buy them for the cheapest price it has to be three months from the moment you look.
(Even then the cheapest tickets can be unavailable if you do not book at the second they become available).
Compare prices for tickets Amsterdam to Brussels with prices for tickets Amsterdam to Paris, and also compare prices for tickets Brussels to Paris. (Or use Antwerp if you prefer that.)
For the leg Antwerp to Brussels, do not buy tickets for the fast trains but travel on the normal trains which are regular and likely much cheaper.
The Thalys sells tickets with compulsory seat reservation, with a coach and seat number given, and are only valid for one train. They do not allow you to break your journey and continue on the next train.
Alternatively, which is the option I would use, you can buy tickets Amsterdam to Brussels for the normal (non highspeed trains) which can be used on all a non-highspeed trains running that day, but only once, and which allow you to break your journey.
Use one of those to get from Amsterdam to Brussels, making sure you arrive early enough to catch the fast train for which you booked your ticket.
I would use the Belgium railway site to research these tickets as they will show you the most options, you may want to switch between their international and their national version, both available in English.
How much you need to pay mostly depends on when you buy the tickets for the fast trains. If you go for the normal trains tickets between Amsterdam and Brussels, those have a fixed price, no seat reservation and no guarantied seat on busy trains.
About prices, the best I can find now from Amsterdam to Paris by fast train is €35, going up for trains nearer the date you buy Brussels to Paris €29, going up for trains nearer the date you buy. And for the normal trains Amsterdamd to Brussels, fixed price €19. All these prices are one way.