Need help with Swiss pass and eurail

Hello, I'll be visiting switzerland in AUG for 3 days,arriving at Zurich by plane and leaving from Interlaken to Paris by train. I haven't bought any railpass and swiss pass/HFC yet. My itinerary is :
D1: Zurich to Interlaken

D2:Interlaken to jungfraujoch

D3: I dont know where to go yet...(any suggestions?)

D4: Interlaken to paris (planning for earlier train)

I would really appreciate it if anyone can give me an advise as to what to buy and the most cost-effective way that is applicable with my itinerary ? On DAY 4: If I get a half fare card, will it cover half of the fair on the swiss part of my trip from interlaken to paris? Or do I need to get a separate fare from Interlaken to paris alone.

By the way I'm planning to include France in my Eurail pass, so the French part of the trips is covered. Thank you in advance.