Interlaken is a good place to buy Swiss Army Knives. One of the best shops is the "Swiss Knife Center" Höheweg 125 as it stocks the full range of Victorinox and Wenger Swiss Army Knives. Other knife shops in Interlaken are "ALSCHER", "Bühler Chalet Shop", "Royal St Georges Gift Shop", "Gallerie Metropole". Most of the mentioned shops engrave text on the knife for free.
Remember to pack swiss army knives in your hold luggage.
Chocolate
You best buy swiss chocolate at the coop supermarket one is just by the ost train station; there is also a large coop supermarket in unterseen, a town right near the interlaken west train station where you have a very good choice of lindt and cailler chocolates, toblerones, etc. at much lower prices than you'd pay in the souvenir shops.