If you only get one stop alone in Houston, make it Hermann Park (free, 445 acres, open 6am-11pm). Next on our list are Discovery Green (free, 12 acres, open 6am-11pm) and Menil Collection (always free, Wed-Sun 11am-7pm); the other three, down to Buffalo Bayou Park, are below with where to book.
160 acres, 300,000+ bat colony
free Tuesdays 5-8pm, adults $25
free, open Tue-Sun 10am-6pm
always free, Wed-Sun 11am-7pm
free, 12 acres, open 6am-11pm
free, 445 acres, open 6am-11pm