You may want to try to squeeze in Writing-On-Stone. Depending on what you like I would spend at least 2 days to hike the trails to see the rock art. Too bad much of it has been defaced. I was there in the early '80's and again last year. I cannot believe the difference and the damage done by people. They have a fiberglass copy of the one wall in the museum here in Edmonton.
If you can fit it in go on the guided walk in the closed part of the park. You may even make it a different trip and take in Cypress Hills and Dinosaur Provincial Parks.
We were going to go to Tofino this year but may just go to Waterton and work our way north to Grande Cache. With gas at $1.43 in Vancouver and a $275 ferry trip (one way), staying at home is looking like the way to go.