I check the local news website, they have a free/nearly free column of things to do locally. I also learn a lot from word of mouth & from paying attention when out & about. In fact, last night when I took the kids to play mini-golf, I learned that they have "kids play free" every Tuesday. Guess who'll be hitting up that deal this summer? lol
I check constantly for things that are low/no cost for us to participate in. Like tomorrow there is an arts fair. We will get drinks at the gas station on the way and go walk around and look at everything and spend nothing. There is even going to be music. We also do alot of camping, going to the park, hiking, etc that is free or nearly free.
sidewalk chalk and bubbles can both be found at the dollar tree. our imaginations are super. There are plenty of parks near by. And if it's cold we go to mcdonalds (after we've had lunch) and order a drink for the adults and water for the kids. So, it only costs like $3.
We have so many parks around our house that we can really try different ones all the time to keep kids from getting bored. I scour different websites like redbicycle.com to find free/inexpensive activities going on in our area