Bernina Express day trip from Varenna

One point on StevsB's plan: Take the earlier 7:24 train from Varenna-Esino station. With the 8:24 departure, you only have 11 minutes to change trains, and you actually have to change train stations. Now they are across from each other on a big piazza, but you will have to walk along the platform, perhaps some ways to the station exit. Then eyeball the Rhaetischebahn station and walk to it and walk out to your assigned carriage. If you have not already printed out your ticket and reservation, you'll have to get those. And while Swiss trains run pretty much like clockwork, Italian trains might not be quite so prompt.

So take the early train, and instead of stress, you'll have time to stop at one of the snack-bar/pizzerias next to the Rhaetischebahn station and have a relaxing cup of coffee and a cornetto.

Thanks for the advice!