I don't know what your plans are but I wouldn't get too fixated on Whistler for mountain biking. Plenty of other good spots so if Whistler is a bit of a detour then other places may be as good – especially for XC type stuff. They might also be slightly more discrete in a "Oh look dear; theres a bike hire place; do you mind if I rent one for a couple of hours?" kind of way. You are unlikely to be able to just accidentaly stumble on biking at Whistler.
Having said that good fun can be had by renting a bike from the places at the bottom of the hill and spending some time in the bike park. For a more XC orientated area the Lost Lake trails are excellent and would keep you entertained for most of day.
My only (may be obvious) piece of advice is to check that what ever bike you rent you can switch the brakes round on. Mid-way down A-line is not the place to discover the brakes are reversed.
I think (but don't know) that you can do kayaking on the lake shore amongst other activities. There is always hiking and (again not sure) I think you can take the lifts up in summer and walk around from the top?
On a separate note: Canadian distances are big so if you're fly driving don't plan on going too far at any one time. Speed limit in National parks is 90 km/h ; most locals do 100; outside of parks 115 is probably realistic.