White Rock Lake and the Arboretum would be decent choices. If you have the transportation, I highly recommend an expedition to Fort Worth's arts districk. The Amon Carter museum is a must see for any serious photographer. The Modern Art Museum and the Kimbel Art Museum are also major world class museums. While there, visit the Japanese Garderns.
The arboretum is fantastic, we visit there frequently. We also love the science museum, they have a fabulous children's museum that is included in the admission fee. I have 5 kids ages 3 months to 9 years old and Dallas is a great place for kids - lots to do!