Tamarabm Hello Tamara,
I’m from Switzerland.
You have two type of tickets:
— Saver Day Passes, who are not refundable and not exchangeable. If you buy such tickets, it is not possible to travel another day. You have to buy these tickets in advance to obtain the best price. The actual prices are 70 CHF in 2nd class and 88 CHF in 1st class and you can take every train/bus/boat in Swizterzland on the chosen date, except some mountains excursions. Be sure to select “Full Price without the Half Fare Travelcard”:
https://www.sbb.ch/en/tickets-offers/tickets/day-passes/saver-day-pass.html
You could also take a boat from Brienz to Interlaken or from Interlaken to Thun.
— Normal Point-to-Points Tickets. They are refundable until the day before departure, but you can also buy the tickets at the ticket office or at a vending machine in Switzerland, the price will be the same. The prices for one way via Lucerne and one way via Bern only with the train are 126 CHF in 2nd class and 216 CHF in 1st class.
Let me know if something is not clear or if you want more informations.
Daniel