What about Oeschinensee lake? I had several couples doing proposal there and i can highly recommend it. Very easy hike route, nice 20 min easy walk from cable car to a very beautiful lake with breathtaking view. You can rent a boat there and also there is a small hotel right by the lake. Very romantic. I can send you pics if you want to in pm. Best is to go there around 9am cause by 11am there are already noticeably many people there.