A nice way would be to pass via Göschenen and Andermatt. This features the somewhat-famous spiralling tunnels of the St. Gotthard north ramp (including the Church of Wassen), a rack-railway section between Göschenen and Andermatt (through the Schöllenen valley, but only limited view on that section as it is partially in tunnels) and then a journey through the Goms with a lot of small villages and a good view on the alps. The last part is the same route the famous Glacier-Express takes.
This takes 6h12′, with hourly connections. I recommend to take a connection starting with an IR46 as those have one interchange fewer. For the first section (IR46), it is recommended to sit on the right side when facing forwards. For the second(R44), the left side is overall better. For the third (R43), both sides are nice. For the last section (R41), the left side is better.
Be sure to search a connection with the following vias to ensure not to be routed onto the GlacierExpress (which is not fully included in the saver’s daypass): Göschenen - Andermatt - Fiesch - - St. Niklaus VS.