I would check local libraries... They 're all having summer reading clubs with special activities (magician, Critterman, juggling, movies, balloon sculptures, etc. - something different every week, and something different at each library.)
As mentioned, Rave Motion Pictures in Hickory Creek has free summer movies every Tues/Wed morning. Many other theaters are also offering similar deals, so check the theaters nearest you (I know near us, Cinemark, AMC, Harkins and Studio Movie Grill have deals - some, not all locations) (Pages 6 & 7 of our current issue list the rest of the season at 6 local theaters, including Rave... or check our calendar).
The Gaylord Texan may be worth an outing. They're having Summer Fest through September... entertainers like people on stilts, jugglers, mini train displays, etc. It's $10 for parking, but that can be comped with a $25 food purchase. Thursday nights, some restaurants are offering kids-eat-free specials; Friday nights 9:30, free fireworks show. (Check Page 5 our May 15th issue for details)
Bass Pro in Grapevine is having special activities this summer as well... lots of free mini-workshops, fishing around the pond in front of the store, etc. (Check Page 7 of our May 29 issue for details)
Founders Plaza (DFW Airport) is a great outing for a picnic and to see planes taking off and landing. Make sure you go on a clear day, and early morning makes for a cooler outing (think breakfast picnic). (Page 5, Jan 1st issue)
Also think about local pools and aquatic centers. They've got lots of great splash areas, etc. Some have admission prices of as little as $2-3. http://www.familyeguide.com/index5.php?id=13&sid=68
Obviously, most of what we feature is south of you, but use us for inspiration. Chances are, if the Lewisville and Flower Mound libraries, rec centers, movie theaters, and businesses offer certain programs, Denton will have similar options.
www.familyeguide.com Your free guide to affordable family activities in Southern Denton County.