Well, for Brussels to Bruges you don't have much of a choice (in a good sense), because direct InterCity trains run few times per hour during the day, journey from Brussels Central is just less than 70 min (although watch out for the fact that there are also trains to Bruges that go a bit curvy route (not directly via Ghent), those take more 2 hours, so make sure you take the one via Ghent). Belgian InterCity trains are pretty good, so no need to worry about comfort. Money wise best option would be to travel during weekend, that way you can get special
Weekend return Ticket With Brussels to Amsterdam case everything is a bit different, because here you can take either high-speed
Thalys train (one-way tickets from 29 euro per passenger if purchased in advance) with 1:50 travel time or use slower
InterCity (full fare one-way tickets cost 42 euro) with travel time of 3:10