Two days in Switzerland

I have exactly 48 hours in Switzerland. I reach Zurich on morning of 30th may and then flying out on 1st June. Any pointers on how to maximize it the best?
I was planning to take a 2 day swiss pass and then use it for bernina express. Is the train journey worth it during May end worth it?

Taking the Bernina Express would be perfectly valid. You only need to check how to get back from Tirano at the end of the Bernina Express journey to Zurich airport to fly home.

May 30th:
train(every hr) from ZRH airport to Bellinzona (luggage lockers at the railway station), visit the 3 medieval castles, go on to Lugano (boat trip on Lake Lugano or gondola to a view point, visit of a subtropical botanical garden, etc.) and sleep there (Hotel Montarina or so).
May 31st:
Bus to Tirano, Bernina Express to Chur and train to a place close to ZRH (airport area if your plane starts early in the morning)

As always meckerdv got you a perfect answer for your 48 hours stay in Switzerland.

About the two day-Swisspass; it might be not that great; you'll have to compare:

The normal ticket is CHF 184.-- in second class and CHF 304.-- in First Class for the above trip. And you'll have to book the bus and the Bernina Express in advance (Bus leaves Lugano at 10 am and you'll be back in Zurich around 8 pm).

Booking pages are the official:

https://www.sbb.ch/ticketshop/b2c/sprache.do?en

or

http://www.rhb.ch/Ticket-Shop.1103.0.html?&L=4

Thanks a ton. This is really helpful and exhaustive. I think you have just carved it out perfectly for me. Thanks one again.

You're welcome. Have a nice trip!