This lake is a true swiss gem! Sadly it's not a secret anymore and the lake tends to get crowded. We visited Seealpsee in the early morning and before 12 o'clock it's totally fine with the amount of people. Afterwards... I had to say I was a bit shocked by how crowded it got, as you rarerly see that with places in Switzerland. So if you want to experience it's full beauty, better be there early!
At the lake, you can swim in there, rent a small boat, use one of the grill places or just relax by the lake. There is also a restaurant by the lake (well actually two) we went to the Berggasthaus Seealpsee.