The route to Lac Blanc is perfect for a 2-3 hour hike and has fantastic view across to Mt. Blanc and to the left, the Mer de Glace glacier.
The start of the hike is very accessible from the Flegere gonodola but is also very popular. Heading up on the first gondola and avoiding the weekends is the best way to enjoy it without too many other people around. Beyond Lac Blanc you can take a circular route back via the other smaller lakes where it can also be a little more peaceful too.
A more challenging route is the Lac Cornu circuit, with access via the Brevent gondola. A tougher route but often less people on the trail.