There are a lot of family-oriented folks here, and a lot of great down to earth advice, so...
I live in Southwestern Ontario, in London. The winter blahs are setting in, and we would dearly love to find a 3-day mini vacation. The downside is that we have four kids, aged 7 - 13, so affordable is key.
Here is a map, showing the area that's on the table -- anything an easy one day drive.
(ie: the 400-500km distance is the practical max)
We've been to The Great Wolf Lodge, and Kalahari. Both are great indoor waterpark hotels, but they're also on the pricey side (when you add on the 3rd and 4th child...). I grew up near Niagara Falls, so that is old hat.