Bexuyett70
Basically you have to select a return journey, as you want to return to Zurich on the same day (I prefer stating the obvious). We’re talking only of half fare 1st class prices, nothing else.
If selecting single journey (if you wanted to stay overnight in Grindelwald), the saver day pass for 66.00 is also displayed as option, because it is (today) cheaper than the regular P2P ticket for 71.90.
If selecting return journey, the (regular, no saver, full price) day pass for HFC is also displayed for 128.00. The regular P2P return ticket for 138.60 (= 2× 71.80) is not even displayed anymore as option anymore, as it would always be more expensive than the HFC day pass.
But the Saver DAY Pass for currently 66.00 is also displayed as option, because - as the name suggests - it is valid the entire DAY (and therefore would even allow you to travel about 6 times back and forth between Zurich and Grindelwald).
Bottom line: Buy the Saver Day Pass now, if you’re happy with bearing the risks of the very restricted refund options with saver offers (bad weather not being a valid reason/justification for a refund until the day before travel).