I took the kids apple-picking at Frootogo Orchards in Waterdown Ontario. It's a great place to take the kids. It's very child-friendly. Admittance is free. There is a huge (and I mean huge) play area, with slides, swings, ride-on toys, hay bales, sandbox and other toys. There is a corn maze, orchards, animal pens and a pumpkin patch. Children love apple-picking. You can also buy fruit and vegetables and pies. I took my children there many times when they were younger. I have a nice memory of breastfeeding my son there. I went to one end of the apple orchard and sat on a rock in the sunshine and fed him; it was great. I just made apple muffins and the kids helped. They love to participate in cooking and baking.
Other good places to take children that you might not generally think of and that are free include garden centres. I loved going to these because it was such a nice place to go for all of us. Even the air is different there; full of moisture and the scents of flowers. There are often waterfalls or ponds with fish. Some places even have sandboxes. The kids love to run up and down between the trees and smell the flowers and look at the different kinds of plants.
Forest trails were also good, because the children could run (and if you have boys you know they need to run) and it was fun for all of us.
Three great things that keep children occupied include scavenger hunts (inside or out); treat hunts (inside or out); and 'painting' things with water. Give them buckets of water and clean paintbrushes and they can paint the deck or the house or the trees. They love it and it's not messy. You can't ask for much more than that.