I can't say much about Saltspring, although I have ridden there. It's hilly pretty much wherever you ride and you won't have much trouble covering the whole island in no more than a day or two.
In Victoria don't miss the Galloping Goose/Lochside regional trail. The Galloping Goose (named for a rickety self-powered rail car that used to travel the route when it still had rails) trail winds through Langford, Metchosin and Sooke, ending at the site of an old mining community called Leachtown (nothing left there to see unfortunately).
The Lochside trail splits off from the Galloping Goose before it leaves Victoria and goes all the way to the ferry to Saltspring. I highly recommend it over riding the highway to Sidney.
Other nice routes include the seaside route around Victoria using Dallas Rd and Beach Drive (if you follow it far enough you can connect to the Lochside Trail), West Saanich Road for an alternate, but hilly, route to Sidney and Metchosin Road through the rural area of Metchosin (crossed by the Galloping Goose trail in several places). You may also want to try a day trip to Shawnigan Lake via the ferry from Brentwood Bay to Mill Bay. Don't bother will the Transcanada Highway over the Malahat unless you want a challenging climb with lots of traffic.
Most people who visit Victoria tour the Buchard Gardens, built on the site of an old quarry, Beacon Hill Park is nice (you'll ride through it if you take the seaside route) and downtown Victoria, especially the section near the Parliment Buildings and the Empress Hotel, is nice to walk through.
Welcome, I hope you enjoy visiting as much as I enjoy living here.