Amsterdam to Paris on Thalys question.

Have you posted this in the Belgium forum? I ask because my answer is based on travel from four years ago and maybe things have changed. You can use your Thalys ticket to get from Midi to Centraal and vice versa by train (not bus, tram or metro). I'd just exit the train at Midi and store my luggage there and then catch a local train using the Thalys ticket from Midi to Centraal and walk 5 minutes from there to Grand Place.
I just checked my notes from my trip and edited my initial post because I realized that Thalys does not stop at Centraal.

I don't believe the Thalys stops anywhere in Brussels other than Midi. So to do a stopover in Brussels, you will need to get off there, store your bags, go to Grand Place by public transportation by whatever means (could be local train to Centraal), then return to Midi, get back bags, and catch the Thalys to Paris. I believe you will need local transport tickets for this, unless someone knows better.

Yes, the Thalys only stops at Brussels Midi. From Midi, the easiest way to reach the Grand Place is to take the Metro from Midi to Bourse, which is in easy walking distance. You will need to get new tickets for the Metro.

I believe the tram is also an option.

I said "Metro" to avoid confusing the OP as the signs say Metro. The line from Midi to Bourse is trams running in Metro-style underground tunnels.

Thanks for the help, this clarifies it for me.

Also, I didn't post in the Belgium forum because I knew I would find the answer here and that forum is not quite as busy as this so questions can go unanswered.