Because Refuge Porte du Plan du Lac is so close to Bellecombe (2307) - where you can leave your car - it's the perfect destination for a family hike. A free shuttle bus (called navette) even goes all the way to the mountain hut, so a member of your party who is not up for the walk can join you there. But there's really no reason not to walk from Bellecombe, it's a gentle stroll. Very suitable for (small) children. And on foot the views are always just a bit better than from a bus.
The refuge is a nice place for lunch. Plenty of space for a lazy picknick. And lots of options for extending your walk.
Parc national de la Vanoise is one of the largest (and oldest) national parks in France, very accessible and well worth a visit. The mountains offer nice views and fresh air. The Vanoise is known for its flora and fauna, and if you can't see the Alpine marmots (and you probably will), you'll most certainly hear them. If you're lucky you can even spot an eagle or Alpine ibex.
Farmers still herd cattle and sheep in the national park. On the way to Bellecombe you'll pass a small working farm (called La Femma), where you can stop and taste and buy cheese (also a really nice blue cheese called Bleu de Termignon).
Refuge Porte du Plan du Lac is open only in summer, from the start of june to the end of september. The free bus only operates in july and august.
Bellecombe (2307) parking. In Termignon follow the D1006 to Lanslebourg-Mont-Cenis. Just after leaving the village, take the D126 before the road makes a very sharp turn to the right and starts climbing.
This hike is suitable for (almost) everybody. The path along the east side of the small lake is a mountain trail, you'll walk on rocks. The way back (along the west side of the lake) is a track. The bus takes that route, so if you're not sure, you could take that track both ways.
Just follow the path to the refuge. At the end of the car park you'll see a sign. From the mountain hut, you can see the path back to Bellecombe.
Just to be sure, don't just take your picknick blanket and lunch. Wear sturdy mountain boots and take some warm clothing and a rain jacket. And don't forget: Slip-Slop-Slap. Take a shirt with long sleeves, put on sunscreen and bring a hat to protect yourself (or others) from sunburn.