Updated:
Take a boat along Lake Thun and make Spiez your lunch stop. From the deck you can follow the wooded slopes and pick out castles and vineyards along the shore. A short stretch is enough for a first outing; you can spend the rest of the afternoon by the bay.
The walk from Spiez pier to the railway station is uphill. Check the bus as well as the boats before deciding how to return. Sailings change with the season, so use the timetable for your date. Bring a jacket for the deck; ask the operator about boarding if you need step-free access.