This little lake is just below the Fürenalp restaurant. It has a beautiful view of the Engelberg valley, which you will quickly leave behind once you start hiking. So it's worth this small detour before heading out on other trails.
There are a couple benches next to the lake, a quiet spot for reflection (pun intended). See location on map.
To get here, ride the cable car up to Fürenalp. Then follow the path labeled "Spiegelsee" (aka mirror lake). It's a loop, descending a bit below the cable car station, then back to the restaurant. Walking time about 10-15 mins.