BC Ferries expensive......only if you consider an arm and a leg expensive.
when I came across with my last move, it was almost $200.00 for the trip. Made living on the island pricy to do the trips out. Good thing was that there was always lots to do on the island.
Butchart Gardens will take a good 1/2 day, but will be really boring for a 13 year old. its a nice garden, but.......
downtown Victoria has a bunch of neat museums(big National geographic, wax etc), undersea gardens, butterfly gardens, and a miniature world.
In reality it depends on what you want to see, your hobbies and interests.
example - if you are into car racing - that would be a great weekend to go onto the island to the RV park that has the
racetrack then you can take the time to visit downtown another day, let Grandma spend a whole day at the Garden while you let the boy expend a bunch of energy by wandering around many of the parks nearby, or fish in some of the small lakes, or batting cages, go carts, etc etc.
There are a number of things to see and do - there are tours available at the local Navy Dockyard, and a Navy museum as well.
here is another website with Victoria calendar.
Not trying to sway you either way, but I know Victoria well, so I can give plenty of ideas.
it also offers up the opportunity to leave the island via the other ferries to do a different path down the coast.