The best way to reach Spiez is by bus 1 from Thun, remember to sit on the left so you get a great view of the Bernese mountains around the Thunersee. When you reach Thun, you’ll be able to see the whole town from the bus stop at the train station as everything is downhill from there.
The gradient is rather mild as the road loops around to the bottom. The Thun castle right by the lake is surrounded by restaurants and hotels and is frequented by Asian tour groups as a scenic destination. To enjoy the lake even more, I’d suggest taking one of the two daily ferries that go across the whole lake.
