Book an apartment or home
Most rentals are cheaper than hotels. You would either go out for dinner, or cook your own meals. There are large supermarkets in Interlaken that make shopping and cooking part of the fun. There are lots of restaurants in Interlaken as well.
The landlord or rental company will provide a free guest card upon arrival. The card allows free local traveling and discounts on local attractions. If you own a rail pass, you get free traveling in a much wider area and possibly a better discount on attractions.
Interhome offers thousands of holiday homes and apartments across Switzerland. They offer discounts on both early and last minute bookings. That makes the Interhome website a good place to start your search:
https://www.interhome.com/switzerland/: all holiday homes
https://www.interhome.com/switzerland/early-booking-discount/: find the discounts for early bookings
https://www.interhome.com/switzerland/last-minute/: find the last minute offers