Luggage and bags:
A back pack with water and other edibles that you would like to take inside. Also carry a small portable flash light as it can get very very very very very and very dark if the light goes out.
Clothing/Shoes/Weather Gear: During summer the weather is quite hot so wear light cotton clothings. Inside the mine it is cool, but not that cool. In winter what ever jacket or sweater you wear will suffice. Also wear thick sole rubber shoes, e.g. hikers or joggers. You will be walking on rocks.
Photo Equipment: If you plan to shoot inside, your flash may not be that powerful to light up large chambers. Carry a portable TRIPOD to get a perfect picture, and avoid camera shakes. I pushed my Nikon D70S to 1600 ISO, and still in some areas light wasn't enough (thus some camera shakes visible).