We live quite close to Waterton, so Cameron Lake is a regular day trip for us. We love this lake, it's at high altitude and can be a little cool in Spring and Autumn so take a sweater. The road to the lake is closed in winter.
You can rent a canoe, kayak or pedal thingy, but it is SUPER expensive or you can bring your own. The car park is close to the launch area and so easy to carry a canoe down to the lake.
Take a hike down the lake but watch for bears as apparently they are plentiful in this area although I have personally never seen one here.
The far tip of the lake is actually in the USA and the rest in Canada and if you do feel energetic enough to canoe down to the end, it's worth the trip as there are some great waterfalls and pools to explore.
You can also do a long hike down to Waterton town from here, which take about 4 hours. The water in the lake is too cold to swim in at any time of the year, but there are lots of great places to picnic, hike or of course explore the lake by canoe or kayak.