The Strandbad (outdoor pool) in Klosters is open (and heated!) during summer. Diving boards, swimming lanes, kiddie pool, and a large fun pool
The Eau La La Hallenbad (indoor pool) in Davos is an indoor/outdoor pool and spa. Olympic pool indoors, very fast waterside, kiddie pool, and outdoor heated pool with lots of bubbles! They have massages and a solarium as part of their wellness offerings.
Other:
Hiking trails starting at or near the village of Serneus
Bad Serneus, a mineral pool within walking distance