I love taking natural photos. Just beautiful lighting, simple outfits and a great smile. Part of why this photo looks so good is that it is photographed on film. Hard to beat real film. It does not exaggerate lines and problem skin, like digital cameras often do. Also the color is outstanding.
The key to taking great outdoor portraits is to find a good location with appropriate, non-distracting backgrounds and match the clothes to the background. For casual clothes a park might work, but for something dressier, perhaps a building in downtown Denver.
And to get a great expression, it is important that you have a photographer who will establish rapport and help you with posing, clothing selection and wait for the perfect moment to take the picture.