The"wine train" from Vvevey to Puidoux won't fill up much of a day. It is a short ride. But, it is well worth it. The views across the vineyards past Lausanne are very impressive. If you get the afternoon sunset, and can avoid shooting into the sun (not easy) the vineyrds practically glow in the light.
Lausanne is not a major tourist detination. However, the intrnational museum of the Olympics is there, and genrates good reports from those who are interrested in that sort of thing. Also, the water front at Ouchy has a batch of nice places to stay or eat, and a nice lakeside promenade. It is directly accessible from the main station via the new funicular.
Ouchy is very nice with lakeside activities and views., and has some distinctly over-priced tourist traps. Read reviews before you go
A friend took me to lunch there last year. I'll ask him to remind me of the name of the one we chose, and the one(s) we avoided. [We were happy with Chateau d'Ouchy...turn left as you get off the funicular.]
The funicular that takes you down to Ouchy also goes up the hill above the main station.
If you spend much time in Lausanne, you'll notice that the terrain is a bit steep.
You might look at Fribourg, even though it is not on the direct line for you. I/we have stayed in Fribourg for a several nights, and visited many times. We like the old city, down by the river, which is mostly German speaking. Very photogenic. Lots of restaurants. The funicular down to the old town (and up) from the park, not far from the main railway station, was famous - may still be, although it has been rebuilt. It was a one-car funicular, and the counterweight was a tank of water, whose amount could be adjusted. It was city sewage water, and could only be noticed on rare occasions, depending on the direction of the breeze.
There is bus service from the main station to the plaza(s) in the old town.
I/we have walked 3 or 4 times from the old town through the Gotteron Schlucht almost to Ameismühle, turning up hill to the right (south then west) then over the fields on trails to le Bourgillon and back down into the old town. Definitely something different. Lots of nice views of the city, and the Jura on a clear day, and the trail through the Schlucht is fun.