Interlaken has the highest concentration of souvenir shops in Switzerland.
Watches
Interlaken has two main watch and jewellery shops: "bucherer" and "kirchhofer" which stock all the big traditional swiss brands tag heuer, omega, iwc, rolex. alternatively, most souvenir shops sell some inexpensive swiss-made watches like swiss military and swatch.
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.
Other souvenirs
Other items to purchase in interlaken's souvenir shops are: swiss music boxes, leather wallets and purses, cow bells, swiss embroideries, cuckoo clocks, some inexpensive swiss made watches like swiss military or swatch, switzerland t-shirts, sweatshirts, backpacks, etc.
If you want to purchase a Swiss Woodcarving, go to Brienz about 30 minutes by train from Interlaken Ost, which is the centre for Swiss woodcarving. In Interlaken, woodcarvings can be found for example at the shops "Bazar Tivoli" and "Albert Schild".
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.