Give it carrots, lettuce and fresh water. 
Give it carrots and fresh water, but don't give it iceberg lettuce because it has almost no nutritional value. For green vegetables, spinach, carrot tops, parsley, turnip greens, dandelion greens, or romaine lettuce would be better choices.
You can also buy rabbit pellets which provide balanced nutrition, like Purina Rabbit Chow, for example.
My rabbit really liked bananas, but you can offer only very small amounts of something like that as a treat.
