Speaking of getting wet...
Most of your macro shots are going to be down on the ground and you are going to be down
there too. A water resistant jacket and/or trousers could come in handy. You can spread them out
on the ground for something to lie on so you won’t need to worry about getting your good
clothing all wet, dirty and muddy. And if it should happen to rain, you are all set.
At the very least you should have a few plastic garbage bags in your camera bag. You can use
them to lie on when you are down on the ground. Plus, if a sudden rainsquall should pop up, you
can throw your whole camera bag into one of them and protect your gear.
On the subject of clothing, be sure to take some sort of hat with you. Often, the sun will be
beating down on you all day and you don’t want to get a bad sunburn. Even when it is overcast
and cloudy, the harmful UV rays will still get through.