I assume you are referring to the RhB in Canton Graubünden in eastern Switzerland. I can recommend the whole railway, it's scenic, & very interesting from a railway operating perspective & has some of the funkiest rolling stock around. I was based in Chur for a week and traveled on every line open to passengers. The highlight for me was a "footplate ride" on a CoCo electric loco from Chur to Saint Moritz. The kicker here was it cost me 800 CHF! But it was the culmination of a 40 year dream, so what the hell, I spent the money.
If you can only devote one day to the RhB then I have two suggestions: 1) The Bernina Express from Chur to Tirano in Italy. you can do it as a round trip from Chur or you can continue on to Milan [not on the Swiss Pass].
2) If you don't take the Glacier Express you can take local trains over the same route to Andermatt, Brig or Zermatt, that will be a long day. You will have to change trains at Disentis, as the RhB & MGB cannot run through as one is non rack & the other is all rack, so locos MUST be changed here. [The Glacier Expresses are the only through trains, they have a straight loco swap]. At Andermatt you could take the Goeschenen Bahn down through the gorge to Goeschenen. From there you are on the old route from where you can get connection to anywhere in the country or you can return up the gorge and continue on to Brig or Zermatt.
I spent 4 weeks in Switzerland in 16 & a further2 weeks in 17 and it was not enough, so I am planning another 6 week trip in 2024/5.