Leave whatever you want really. You can pour water on the ground as an offering. I am assuming the offering is to nature? I sprinkle cornmeal as an offering when I take something from nature, like herbs, rocks, shells, ect. You don't have to give an offering, but some people feel the need to. If you are giving an offering to a god/dess, you could give them some candy, a drink, or an item. You are still not required to though.
myself, it's the leftover cake and wine from the circle, poor it on the ground outside, the liquid is absorbed by the plants and the animals eat the cake. but it could be anything, if you want to, say, tie a ribbon on a tree, you could.
Along with everything that is said, you could always leave an offering of blood. Blood is quite powerful and rich with your own energy. Along with that you could mix blood with wine. You could also leave meat, if your diety is not against killing animals. You could also make them a meal. Anything is possible as long as you believe it will work.