Luggage and bags:
You will need a comfortable backpack for the tramp. Probably about 30 Litres maximum for any overnight trips and a daypack should be fine if you plan to do the one day hike. It might be good to consider getting the drinking bladder's installed in your backpack so that you don't have to stop and get your drink bottle out.
Clothing/Shoes/Weather Gear: In New Zealand the weather can be extremely changeable so it is recommended that you are well prepared for any tramping adventure. This includes the one-day Tongariro Crossing!!! Please make sure that you have clothing to suit the conditions and remember that just because it is sunny and warm when you start does not mean it will remain that way at the top of a mountain. People, usually tourists, die every year on New Zealand mountains because the are ill prepared for the changing conditions.
You will need comfortable but sturdy tramping boots with ankle support, wool socks, thermal underwear (top and bottom), polar fleece or wool jumper, wind and waterproof jacket (such as Gore-tex), gloves, hat, trousers (not jeans) and shorts. If you are camping on the mountain make sure you have a warm sleeping bag
Toiletries and Medical Supplies: It is good to take toilet paper with you up the mountain as the toilets do not always have supplies (especially in busy times such as Easter). There are only two places with toilets though. Please make sure at least one member of your travel party is carrying a good first aid kit. I bought a good one at the Kathmandu chain of stores in NZ which was desgined for trampers.
Photo Equipment: You will certainly want to take pictures up on the mountain. I took a compact digital camera. I saw a few people with larger cameras but remember that you have to carry the camera up some steep hills so you might regret taking anything too big.
Camping/Beach/Outdoor Gear: In most cases you will need to carry your own food and sometimes cooking equipment too if you are planning to stay on the mountain. The huts have gas cookers in them but we carried a small hiking stove with us and had miso soup in one of the craters for lunch which was nice!
Luggage and bags:
A comfortable back pack is always essential. If you are going for the day only, a day pack in which to carry clothing, water and food is all you might need.
If you are doing this walk as part of the whole round the mountain walk, obviously a larger/bigger volume pack will be more suitable.
Clothing/Shoes/Weather Gear: Make sure you have well worn and sturdy footwear/boots. Do not try and walk this track in sneakers or (horror of horrors) as I saw one Asian tourist try, dress shoes - no I am not kidding.
If you have bought new boots, make sure you wear them in BEFORE doing the walk otherwise you will be in agony.
Even if its a sunny day when you start, make sure you have thermals, a warm hat and waterproof clothing. The weather tends to deterioate as you get higher and there is usually a cold wind.
Toiletries and Medical Supplies: Sunscreen is important as the sun is especially fierce at these altitudes.
Photo Equipment: Well, what can I say - every good VT'er always carries a camera. Just make sure you are able to waterproof it in your pack or day bag as the weather can turn pretty quickly.;
Camping/Beach/Outdoor Gear: A walking stick of some sort might be a good idea. It helps steady you and is very useful on the long downhill path and steps on the other side.
Miscellaneous: Kathmandu Clothing stores are a great place to start if you need to add to your collection. They have a great range of clothing and footwear and the staff are very friendly and helpful. (and no I don't work there :)