I am buying a train ticket from Paris to Amsterdam (Thalys), can I hop off in Antwerp to spend sometime with a friend and then hop on again in the same day (within an hour or two)? or do I have to buy a separate tickets?
Thalys is an all reserved train service, and your ticket is only valid for the train booked.
If you want to spend some time in Antwerpen just buy a Thalys ticket from Paris to Antwerpen, and buy another ticket for Antwerpen to Amsterdam.
Between Antwerpen and Amsterdam you have the choice between two services.
Thalys (which again you need to reserve, and commit to a train).
Intercity. This is a non reserved, turn up and go service. Buy tickets at the station for this.