T.O.
I used to look at Guidelive.com for free entertainment and parks to go to with the kids. Not sure if they still have it but they used to have a top 100 free things to do in the Dallas area.
Here's another site I found...
http://www.visitdallas.com/visitors/free-things
And if you like looking at older buildings with a big area for running around, check out the Farmer's Branch Historical Park. It's free, but if you ask the docent to let you in to the homes that aren't already opened, then you should tip.