It depends on the day. If I am playing a tournament and using my staff bag, I will carry at least a rain top. If it is spring or fall I will add a pullover vest. If the weather looks like it may be bad, it will be the full rain suit, extra hats, a few extra gloves and spare socks.
If it is just a casual round with my carry bag, it has only the bare essentials in it. Rain gear only goes in if the chance of rain is pretty high.
Usually never more than 5 or 6 balls and 1 extra glove.
Snacks are a must, never want to be hungry out there.