C.M.
Try the Rockin R Ranch in Frisco. They do trail rides for $35 and up. I got the brochure at my son's school and was planning on taking he and some friends out. (I took lessons when I was a child and want my kids to be familiar with horses...). It's the only place I've found in the area other than boarding stables (which are all over DFW). In most cases, the boarding stables are only for people with their own horses.
Oh - and also water park related - Hawaiian Falls in Firewheel is fun, and MUCH cheaper than Six Flags Hurricane Harbor (and you can bring a cooler with your own food).
Have Fun!
C.