The Globe & Mail has a Saving Money section in which it recently published tips to same money on a summer trip.
As I searched for a link to the article, I came across the online questionnaire they use to gather information. The article was a summary of the 23 comments they got from experienced parents. Check it out! It seems like camping is a favourite.
If you're new at camping, I recommend you try Oastler Lake Provincial Park near Parry Sound. The 3-hour ride from Toronto is very pretty in its last third. The park is small and easy to manage for young families (preferably with kids 5 years and older).