The answer depends on what you have, obviously. But by the sound of it, boots are a must.
Up to you whether you wear sloppy-joe socks with that or proper long socks (with or without flashes).
As for the kilt, if you have one in a hunting colour scheme, that would probably be best, but anything short of an arisaid dress tartan would seem acceptable. An arisaid would likely show dirt pretty quickly.
Top that off with a casual shirt (don’t feel obliged to make it khaki), and a day coat over it. I have a brown corduroy jacket that I plan to have converted to kilt use when I have a kilt – it would probably serve.
But a bomber jacket or any short zip-up would probably work.
Bonnet? I think for the evening, at any rate, especially since everything will be out of doors. But again, daywear, not fancy.
Tie or no tie? Up to you, but again nothing fancy.
Regards,
Mike