If you’re having a location wedding; in a park, on a lawn, on the beach, let your guests know (in a nice way)that they should plan accordingly. Add a line to the end of the invite (where you might have said, “Black Tie Optional”) that lets them know that “Stilettos will sink” or, “You’ll be walking on shifting sand”. One of my clients had a basket of black and white flip flops at the entrance to the ceremony. (And the bridal couple both wore flip flops too; hers trimmed with flowers!
(The picture is actually a cool note card from JustforyoubyJen.com, an artist who makes very neat stationary; wouldn’t this make a perfect casual invite?)