Best choice for a day trip

Day trip from where?

Bellagio, IT
Already been in Italy for 3 weeks. Have an extra day so was thinking about a day trip to St Moritz or Locarno-Lugano

Jim, Again, where are you starting from, that does affect where it is sensible to go.
Locarno and Lugano are two different places, some distance apart. If you like lakes, I suggest Stresa on Lake Maggiore, with the associated Borromean islands.
Or Lugano. You can spend ½ day in the town, and then either do a ½ day boat trip around the lake, or go up the local mountain (I forget the name).
It partly depends on what you have been doing up to now, and what would be something different.

Thank you Chris, this is helpful. We will be leaving from Bellagio, IT.