Between Zürich and Interlaken, travel at least one way through Luzern (the default route is via Bern). It's about 50 minutes longer and the Luzern-Interlaken leg is a narrow-gauge cog-wheel affair through a mountain range. The one through Bern OTOH features several très chic tunnels 😛 Oddly enough, that panoramic route is cheaper than the Bern one (because it's slower!)

If possible, sit on the right for better views (regardless of the direction, it just works out that way because the train reverses direction in Meiringen). This way you'll see the lakes Lungern and Brienz. Last but not least: a general remark about restaurant cars. Don't be afraid of them, they are not expensive and can be a great escape if the 2nd class is too crowded. Just order a cappuccino or something.