Our family loves to go to Jasper, we usually hit it for at least 4-5 days every year. The Whistler campsite is great, however if you like to have a campfire, they do not have full service with a fire. They do however have Electrical with a fire.
There is a campsite across the road that has full hookups as well called Wapiti, their full hookups are in a parking lot. Not exactly the camping experience.
If you are up for a good hike, the Old fort point is a nice hard walk (lots of elevation in short distance, but if you are in good shape it is doable) and takes about 2 hours. It has a fantastic view of the town with mountains behind. I will see if I can post a picture of it in the next few days.
There is also Lake Edith Cavell, which is close to the Marmot ski area, which has a great walk to a glacier lake, which if you are lucky like I was 2 years ago you can see a large piece of glacier fall off into the lake. It was breathtaking.
Anyway if I think of anything else I will post it.