There is tons to do and see, i’ve been here a year and still got a lot of things to tick off the list.
The weather should still be good and pretty hot (compared to the UK) in September. There are the usual things like Stanley Park, Granville Island, Gastown, Chinatown, East Hastings Street (its a slum but you sort of have to see it, don’t stay there though), Canada Place.
If you feeling fit and want awesome views of the city do the grouse grind, basically a hike up grouse mountain which will take you around an hour or so if your reasonably fit, $5 to get the skyride back down, otherwise get the skyride up (around $35 each), you also have the option of doing the new zip lines up there and theres the captive grizzly bears which are cool to see up close. You can also get good views of the city from the road up cypress mountain
Another good hike would be the chief in squamish which is also next to shanon falls.
A night or two in whistler is a must even if you weren’t riding but as you may be interested in riding you might as well do it there. lots of little trails around lost lake if the bike park isn’t your thing and its just cool to hang about the village watch the bikers or go for a wander round the trails.
You could take the ferry over to victoria for a day or two.
for shopping, Robson Street is the main one downtown, if huge malls are your thing then metrotown in burnaby is worth a visit, its accessible by skytrain from downtown.
Maybe pushing it a bit but I did a four day road trip out to the rockies and back, it was almost all driving with the highlight being the icefields parkway, we covered about 2100k in the four days but it was so worth it. stayed in a few different places along the way which i could recommend if your interested. you’d obviously need to hire a car and fill it up quite often.
I’ve probably missed a ton of things of that list but its a start.