Updated:
The warm saltwater pools at ERMITAGE in Schönried are tempting after several days in walking boots. There are indoor and outdoor pools, with saunas too. Reserve a day visit and give yourself a few hours to soak and rest; Schönried also takes some travelling from Lake Thun.
Day-guest places are limited, with separate hours from hotel residents. The spa can confirm a date and which areas it includes. Pool and sauna rules differ, particularly for children and swimwear, so settle those details when you reserve.