Shallow Depth of Field
A creamy lively-green background looks great and if you have appropriate focal lengths (fast telephoto zoom lenses) and lenses with wide aperture, you can create dreamy pictures.
Since it's an outdoor wedding, the venue is likely smaller and setup in a way that is easy to move around. Use this to your advantage and capture the ceremony from a variety of unique angles, behind, to the side, and in front of the couple. It is always best to give your clients a variety of shots, unless they have explicitly stated what shots they like.
The Decorations and Detail
The decor in an outside wedding is different than what you would normally find in a conventional church setting, use this to your opportunity and photograph the carefully-placed decor and your clients will love it! The fact that you took the time to appreciate their attention to detail will be noticed.
Go Wide and Low
Capture the details in the lawn chairs and ceremony by capturing the bride going down the aisle with the grass in the foreground and chairs to the side.