Walking in Canada’s national parks?

We’re going to Canada (Vancouver-ish area) on holiday this summer and I was wondering how well sign-posted trails are in their national parks (Banff and Jasper particularly). Can we just turn up or do we have to buy maps before hand?
Also any top activities you can recommend in Calgary/Banff/Jasper/Whistler/Vancouver Island will be appreciated!
Thanks

You may find some useful info here:

http://www.canadianrockies.net/bnpdandw.html
http://www1.travelalberta.com/en-ca/index.cfm?pageid=76

Have a great time!