Swiss trip suggestions - 4 days during Mar-Apr 12

Dear friends,
I am travelling for 4 days with family to Switzerland from Paris starting from March 31, 2012. During my trip to Swiss, my plan is cover Interlaken and Lucerne with excursion trips to Jungfraujoch and Titlis. I would arrive Geneva at 10:27 hours in the morning on Mar 31.
My plan is to go for the 4 day Swiss pass. However, I just came to know a few days back that Swiss Pass 2 for 1 offer would start just from April 1, 2012. So, i am just thinking if i should buy another P2P ticket from Geneva to Interlaken and travel on the same day (March 31) or should i just stay back in Geneva for a day, see a few places and leave Interlaken the next day (so that i can cover that leg of the journey for free under Swiss Pass).
I would prefer to stay back in Geneva really as i would save 292.5 CHF on P2P fare (260 CHF for 4 adults + 32.5 CHF for child) and travelling to Interlaken the next day. However, i think i am taking the risk of travelling to Interlaken and Jungfraujoch on the same day which i dont know is possible from Geneva?
If i have to travel the next day to Interlaken, then i guess that i need to board an early morning train to Interlaken and keep the luggage in the luggage locker in the Interlaken station itself so that i give enough time to myself and family to spend the day on the Jungfraujoch excursion. If i need to avail the luggage facility, then do i need to check the "Baggage Switzerland option" as mentioned in http://www.sbb.ch/en/station-services/services/baggage.html
Look forward to your kind advise.
Thanks and Regards
Gautam

Dear Gautam,

I can understand that you don't want to spend the extra money on full priced tickets. If you travel to the Jungfraujoch from Geneva, you will not be able to reach the summit early in the morning. But you would still be able to get there before 10:30 if you take the very first train from Geneva as shown by the timetable. It can be done if you don't mind to get up early. You don't need to buy any luggage services if you want to use lockers. Just put some coins in any available locker to use it. But if you want your luggage to be transported for you, then indeed check the page you're referring to. You can arrange for it at the Geneva rail station.