We have been to Kandersteg 5-6 times, always without a car.
Walk through town to the gondola base station and ride up to the lake (Oeschinensee). Or hike up, but it is worth the ride to have more time to hike when you actually get to the lake. Two wonderful hikes; our favorite is the one to Fruendenhuette.
https://www.fruendenhuette.com/
You can find more information on hiking and other activities on this page:
https://www.oeschinensee.ch/en/
The Rodelbahn is popular, or you can rent rowboats and go out on the lake.
Another fun thing we have done with teens is rent bikes at the train station and ride into the next valley, the Gasterntal. The valley is undeveloped, apart from farms and a rustic inn or two, and the road in is private, so there is very little traffic. for the first part you are on a narrow road carved into the cliff, above the rushing river.
https://alpinetraveler.com/gasterntal/