Sarah,
Here's a link to my galleries:
Rockin-Photos - Photography by Jerry Bishop
Look at the ones marked MERCER ARBORETUM. It's near I-45 and FM 1960. I believe they close at 7pm during the summer, so if you're starting at 6pm, you won't have very long to shoot there. Also, I have more photos taken in Mercer in there. See "Cosplay...", "Sisters...", "April Mini...", "Portraiture...", and "May Mini...".
Jesse Jones Park in Humble has some very cool old buildings, but you have to walk about 1/4 mile to get back to them. Photos taken there are in galleries marked; "Vampire...", "July Pioneer...", and "Pioneer Homestead...". I believe this park also closes at 7pm during the Summer.
In Kingwood itself, look at the gallery marked "East End Park".
Let me know if any of these help you, and be sure to post some photos once your shoot is done.
Jerry