Jun 16, 2015 - 9:20 AM
One km per day is sufficient to change from trains to buses to cable cars while doing day trips. But changing modes of transport means that one does walk more than when doing things by car, and connections can be tight sometimes.
Yes, I think it would be better to first visit Saas-Fee, spend a night or two, and then proceed to Interlaken. Unless you're fine with quite a long day trip of course. You can't drive in Saas-Fee, but there are buses and taxis: www.saas-fee.ch/en/arrival/#c20 78.
Note that dropping a car in a different country usually is very expensive, so be sure to check that beforehand.
The Swiss Half Fare Card helps you saving on cable cars, boat trips, etc. It's the best option for drivers. This forum discussion might be useful for you too. In your case, I think it's best to combine car and public transport, based on your plans of the day. For example, a trip to the Brienzer Rothorn can easily be combined with a boat trip from Interlaken Ost to Brienz. Once you've visited the Brienzer Rothorn, you can take a train back to Interlaken Ost. The boat dock, the cogwheel train and the regular train are all within 200 m. In this case you don't need a car, whereas on other days it can be the more convenient option.