We stayed in interlaken last week. Just turned up on the Wednesday and found a hotel. Stayed at the Bernerhof - by the station. Its a best western. With hindsight we would have looked for a bit longer but we wanted to make sure we had somewhere.
It was ok, clean, comfortable and not too noisy considering we were on the front.
I also picked up details for the following hotel. Not stayed there but looks lovely. A couple of minutes walk back from the station but close to everything.
www.beausite.ch