Spiegelsee am Hochkeil (literally: "mirror lake") is definitely beautiful and worth a visit if you're already on your way up the Hochkeil mountain. Just don't expect too much - it is actually more a puddle than a lake and if you don't know where it is, you might actually miss it. When taking the trail to "Hochkeil" around 45 minutes in (you can already see the peak), there is a little path going right, off the main trail, and there it is. it really is very tiny, so don't expect to do any swimming or so, but the reflections of the Hochkönig peak are insane and totally worth it. Also, despite it's fame, usually it does not get crowded here.