Shehan Avindra
Google
Undoubtedly, it is one of the most beautiful places in Ontario. Grotto is highly recommended. It is so beautiful the photos can't capture them. Water is crystal clean, and there are plenty of untouched places along the trail you can take a dip. There are multiple trails, and based on your capacity, you can pick which path to take.
Note : You need to have a reservation to enter the park, even for day use. It's pretty difficult to have one when it's closer (particular during long weekends), so make arrangements in advance. Day use will give you only 4 hours of time, which is barely enough to explore this park. To enjoy the full experience, you need to camp overnight.
If you visit the park for day use, don't forget to visit Singing Sands, which you can access for free with the same pass.