Tofino would defnately require an overnight stay. It is pricey, and crowded even off-season, a victim of it's success and popularity. Whistler is the same. (My opinion, I prefer to be more solitary.)
I always prefer when I travel to avoid the spots that ALL the tourists go to and find smaller, but still fascinating locales, so I'm going to suggest other options.
An alternate plan could be to go partway across the Island. There is a wildlife rescue centre in Errington - see eagles, bears, racoons, deer... whatever has needed care. Englishman River Park has a large, noisy waterfall that disappears into a crack, and good trails. Goats on the Roof Market in Coombs is a popular spot. The smoked salmon there is my favorite. Lots of other delicacies. A number of other funky shops, a parrot refuge, and a butterfly garden are nearby. Little Qualicum Falls Park has a number of beautiful cascades, and more trails. I think I like it even better than Englishman River. A bit further along is Cameron Lake, more of a picnic spot, but the highway driving along the shore is quite scenic. At the end of the lake is Cathedral Grove, the only spot on the mid-island with old growth trees. Trails and photo opportunities. If you keep going, over a mountain pass, you'll come to Port Alberni, which has McLean Mill, a forestry museum well worth seeing, that you get to by riding a steam train. Port Alberni is about an hour and a half from Nanaimo, if you drive fast and don't stop, but even a few of my suggested stops would make it into a day trip.
If you want to get to the west side of the Island you could stay overnight in or near Port Alberni (Sproat Lake, say), and take the Lady Rose in the morning. It is a freighter/passenger boat that travels down the Alberni inlet to Bamfield. I've taken it a few times, and have seen bears on the shore turning over rocks looking for crabs every time. Bamfield is a tiny fishing village, perched on an inlet. You can stay a few hours and take the boat back the same day, or stay overnight and return the next day. Bamfield doesn't have the sandy beaches and surfing that Tofino does, but because the road to it is pretty bad, it is not crowded or so overpriced.
Sorry to go on for so long, I love my home and am glad to share my experience and opinions. Hope I get to see you in September.