Lake Marian is and extremely beautiful alpine lake a great hike on the drive into Milford Sound. If you hike this on a warm day, it's not a bad place to go for a (very cold) swim.
The track starts soon after the turnoff onto Hollyford Rd off the Milford Highway. The track begins with a bridge crossing over a river and then soon reached Marian Falls. From the falls the trail climbs steeply through forest until the lake. The lake is a beautiful place to spend several hours and then return via the same track. The track is covered in roots and rocks and requires care as to avoid falling.
A quicker version of the hike would be a quick 20 minute walk to the falls.