We will be in Switzerland for 6 days in October. We are looking at getting the Flex pass for 3 or 4 days travel. Can we use the pass for non-travel purposes (.e.g., museums) on days when we are not taking train trips, or will it count as Flex pass days?
No, you can only do free museum visits on pass days.
What are you doing on the days you do not intend to activate the pass? If the answer to that question is "ride mountain railways and/or cable cars" than you are better off with the Half Fare Card. You do not enjoy any of the advantages of the STP (reduction etc...) on days you have not activated it.
Thanks for the response!