Driving through the Pacific Circle Route in a day, is doable, but it is best done at a more leisurely pace, allowing yourself time to explore and enjoy the natural wonders of Vancouver Island.
The map here has a few suggested stops, but there are many more not listed that are worthy of spending a few minutes or hours exploring.
The route is based travelling north on Highway 1, and returning down Highway 14 but it can be done the other way except with the caveat that the outlooks on the Malahat portion of Highway 1 are only accessible by northbound traffic.
Some notable stops not mapped:
Avatar Grove (large old growth trees as well as Canada’s “Gnarliest Tree”)
Botanical Beach and Botany Bay (tidal pools and hiking trails)
Fairy Lake’s Bonsai Tree (small tree living on a log in the lake)
Harris Creek Sitka Spruce tree (old growth tree very short hike from highway)