You do not have to drive as far as Cathedral Grove to see big trees. The Cowichan Valley is home to huge maple trees, you will find giant cedars and firs at Bright Angel Park with its own small, not too high (free) suspension bridge.
The Cowichan Valley also offers many wineries, golf, whale watching, beautiful hiking, a great out door museum for the train buff in the family. The BC Forest Discovery Centre is an outdoor forestry museum with a steam train.
The food available in Cowichan Bay is quite extraordinary. Local ice cream local cheese, local wine, fresh crab yum.
Victoria is a great city. Beacon Hill Park is beautiful as is Butchart Gardens. You have not said how old your son is. He may prefer Beacon Hill Park with the playground, water park and petting zoo.
A walk on the Breakwater is always a good thing. Stop for a snack at the Breakwater cafe at the beginning or end of your walk. If the tide is out and you are nimble walk on the outside of the breakwater instead of the top.
I know you said you are afraid of heights, but the view from the top of Grouse Mountain in Vancouver is spectacular, especially if you are there to watch the sun set.
Granville Island is in Vancouver and the place to find all sorts of great eats int he public market, as well as theatres, artists shops and more. It is worth a trip just to watch the buskers.
China town in Victoria or Vancouver is worth a visit. Victoria's is the oldest in North America. Vancouver's is larger and really interesting.
Stanley Park is a must see in Vancouver, there is an Aquarium, a walk all around the park and beautiful views.
If you want a nice tall suspension bridge go to Lynn Canyon instead of the Capilano Suspension Bridge. http://lynncanyon.ca/information/ It is free.
Seattle, Victoria and Vancouver are all quite pricy for hotels, especially in the summer. One I like in Vancouver that is relatively reasonable is the Blue Horizon on Robson Street. Big clean rooms with really comfortable beds. If you get a room on an upper floor, the views are great, although you may feel more comfortable on a lower floor.
If you are in Vancouver the last two weeks of August check out the PNE Pacific National Exhibition. Like a state fair there are rides, animals, free (and paid) concerts and of course food.