I noticed that George Bush Park didn't have a letterbox hybrid. So I figured I'd rectify the situation.
Letterbox Hybrid
Letterboxing is another form of treasure hunting using clues instead of coordinates. In some cases, the letterbox owner has made their container both a letterbox and a geocache and posted its coordinates on Geocaching.com. If there is a stamp inside a Letterbox Hybrid, it is not an item intended for trade; the stamp is meant to remain in the box so that visitors can use it to record their visit. To read more about letterboxing, visit the Letterboxing North America web site.