The hike up to Lake Agnes from lake louise is quite a pleasant one in the summer ,at the top of the trail is a tea house where you can reward your efforts striding up the hill with tea and cakes whilst looking at the splendid view. In the winter the views are just as, if not moreso spectacular but the hike up is a lot harder in the deep snow, and the teahouse is closed for the winter so if you are planning tea & cakes at the top you had better stop off at the chateau deli and get a flask made up!
If you are going to make this hike you must be of a reasonable level of fitness, and wearing protective clothing, allow yourself plenty of time ( i would say 3 hours there and back, not allowing for your picnic), but if you are up to it-dont miss it.
The trail is signposted from just outside the chateau at the beginning of the lake, the trail branches away behind the trees up into the hills, it is well signposted and the trail even in the snow is well defined so you cant get lost, the long steep switchback trail stops first at the little mirror lake and then continues for another half a mile up to lake agnes. It is a beautiful trail where within minutes you are hidden from the mass of tourists at the chateau and are in your own little wonderland, once you are at lake agnes you can see the chateau as a little dot in the distance, enjoy your picnic and the wonderful views and then head back to the chateau for a soak in the hot tub-you will have earned it!