As a tourist, you can purchase the Swiss Half Fare Card: Swiss Half Fare Card – Switzerland at half the price | SBB
It offers the same advantages as the resident’s Half Fare Card. You are therefore welcome to buy Supersaver tickets, Saver Day Passes, and regular Point-to-point tickets with the Swiss Half Fare card. And, when using Supersaver tickets and Saver Day Passes, please keep in mind that they are both non-cancellable and nonrefundable. In addition, Supersaver tickets are only valid on the trains as proposed by the timetable at the time of purchase.
→ The only ticket you can’t buy is the Day Pass for 2; this offer is only available for residents.
Your question is frequent and has been answered already in the English-speaking SBB Community: Questions and answers - SBB CFF FFS Community