Hi Sian

......
We went to Canada for three weeks in Summer 2002 and after a 4 night stay in Toronto flew over to Calgary, got a rental car and drove to Banff. I'd HIGHLY recomend taking the Banff Gondola up to the top of the mountain - they have footbridges linking some of the mountain peaks and the views are wonderful! We took so many photos up there and there is also a restaurant at the top if you want to stay a while.
We took the bus from the back of one of the hotels off the main Banff Avenue to the Gondolas, as it would be a very wearying uphill climb, but once we'd taken the gondola back down, we walked down through wooded footpaths to the Fairmont Banff Springs - like a little castle on the mountainside! A word of warning, although this hotel is highly rated (and expensive), the rooms are small and there are a lot of hotels to choose from in Banff.
Lake Louise is a 'must see' and if you do take the path upwards that you'll see many people taking, be sure you have enough bottled water with you and good walking trainers. However, I preferred the nearby Moraine Lake as it is less touristy.
We drove along the "Icefield Parkway" which has stunning scenery and stopped off at the Columbia Icefield where special "Snocoaches" take you onto the glaciers! We also stopped off at the Athabasca Falls.
We went onto Jasper - now I hope I'm not offending anyone on the Canadian board but I have to say that we did not like the town of Jasper at all - except for the wonderful Fairmont Jasper Park Lodge which Oracle mentioned - we stayed in Jasper because it is at the other end of the LONG drive along the parkway and also to visit other lakes and superb scenery in Jasper National Park, but unless I could stay at the wonderful Fairmont, set in superb scenery, I would not recommend it. (My brother stayed in this hotel this year and loved it and so did not go into the centre of Jasper at all).
White water rafting is available in Banff and Jasper (and nost places!), and you can select different levels according to your nerve/experience/physical ability! We also went on a horse trail at Fairmont Jasper Park Lodge.
We drove the VERY long way from Jasper to Sun Peaks - this was chosen as a stopover to break the journey to Vancouver - Sun Peaks is really a skiing resort, and we were advised to stay there rather than Kamloops.
Our holiday continued onto Vancouver............ and then to Vancouver Island and a stay in Victoria where we took a whale watching trip.
In all, I would say that we probably did a bit too much - DH planned the whole thing

and was so determined not to miss anything - and so I would suggest really looking into it closely and deciding what your absolute "must dos" are.
Hilary on the UK boards went to Canada a year after us and she will be able to offer you some great advice - she posted some great trip reports on UK Report board. I believe they concentrated more on Vancouver Island.