Pack List for Day Hikers

Much of this is going to seem pretty obvious to experienced hikers, but there's a few things you may have overlooked if this is your first time in Yakushima. 

Pack Essentials
Backpack: A rain-cover highly recommended, but don't expect the rain-cover to keep your pack dry if it rains sufficiently; instead, take plastic or ziploc bags for your valuables. You can use a garbage bag as a cheap pack-liner.

Breakfast & Lunch: Depending on where you are staying, you can order a boxed breakfast and boxed lunch (called a bento) the evening before the hike and pick it up before you board the bus. Ask at your accommodation. Note that there are no trash-bins at the trail heads nor along the trails.

Disposable Toilet Pack & Toilet Paper: Can't go five hours without a toilet? Then you probably want to invest 250 to 500 on a toilet pack to use at a booth in the mountains as well as some toilet paper, but remember that you'll have to carry the contents back to the trail head with you. Disposable toilet packs are also highly recommended for crowded days and for folks going on two-day hikes. The nicest disposable toilet booth on this trail is shortly after Daio-Sugi (大王杉), before entering the World Heritage Site. There is no toilet booth at Jomon Sugi.

Map: You can get a black-and-white printout of the trail from an information booth. Especially if you are not hiring a guide, please bring a map!

Water Bottle: 500ml~1L is fine for most day hikes, but you may need more if you sweat a lot or if the weather has been dry. You can fill up just about anywhere the water looks clean. I don't recommend drinking the water immediately below a mountain hut, bathroom, in front of Wilson's Stump (because there used to be a toilet-pit nearby), or at lower elevations around towns. No filters or tablets necessary.

Watch: Watch your time. For Jomon Sugi hikers, plan to reach the end of the train tracks by 10am at the latest and note any bus connections you need to make at the end of the hike.

Headlamp: In the summer months, it may be fine to leave the trail head at daybreak, but if for some reason your return is delayed, you don't want to be walking back in the dark.

Optional/Seasonal:
Light spikes and eizen: If the forecasted low is below 8 degrees, there's a good chance of encountering ice. Light, slip-on spikes work great on the railroad tracks heading towards Jomon Sugi, while light-eizen are advisable for snowy conditions. If you hire a guide, usually he/she will have these for you.

Hiking poles: Poles can be a hindrance if you're not used to using them. If you are young and do not have knee problems, I think it's easiest to have both hands free to grab tree roots and trunks on most trails. However, poles can be quite useful when there's snow on the trail, and they can take a lot of pressure off your knees if used properly. Please put rubber caps on your poles to preserve the trail.
 
Umbrella: Use of an umbrella is not advised while hiking. However, some people do use umbrellas while walking the train tracks from the Arakawa Trail Head towards Jomon Sugi. Please be aware that the tracks can get slippery, and, please, don't use it while crossing bridges! It is nice to have an umbrella for lunch and break times, and in case you have to stop along the trail.

Sunscreen & Sunglasses: Most people are fine without sunscreen for forest hikes such as Jomon Sugi or Shiratani Unsuikyo trails. However, if you are hiking in the interior mountains, you may need these.

Bug Repellant: Especially for hiking above 500m altitude, I don't think you need it. Sometimes there are bugs around toilet areas, horseflies in summer months, and there can be quite a few leeches on minor trails, but I recommend avoiding these areas if you can't tolerate the bugs.

Trash Bag: Use it as a pack-liner, or use it as gloves if someone should injure themselves and start bleeding. Finally, use it to carry out your trash.

Camera: Bring a plastic bag or ziploc just in case. If it's raining, consider taking pictures on your way back from Jomon Sugi. I've seen cameras stop working after several hours of mist. (Even waterproof cameras can be affected by humidity.)

Cell Phone: Just like the camera, safeguard your cell phone from rain. You may get a clear signal around mountain tops, between the Jomon Sugi observation deck and Takatsuka Hut, and a couple other places along the trail. But otherwise you may want to turn it off or set it to "air-plane mode" to protect it and keep the battery from draining as it searches for a signal.

Winter Gear: An axe and other preparations are advised for hikers above the tree-line in the winter, but I do not take hikers when I believe conditions may be dangerous. I've never heard a a successful snowshoeing expedition in Yakushima. Be aware that in such conditions the road to the trail head may freeze.

Clothes
Rain Gear: Think 6-10 meters of rain in a year and 100% humidity. This trail is pretty hike-able in all but the worst conditions, but weather changes fast here, and the weather forecast for Yakushima is not intended to cover the mountain trails. If the a good storm catches you in a garbage-bag poncho, you may feel like garbage. Beware of "water-resistant" clothing, which is not intended for ten hours of rain, or one hour with over ten centimeters! Also, if you are hiking with others, realize that you may have to wait in the rain for others in your group.

Breathable waterproofs like Goretex are highly recommended. If you don't have both pants and a jacket, stop by a rental shop the afternoon before your hike, and pick up a set for 1,000 yen.

Hiking Boots/Shoes: Flip flops seldom last the whole way, and they're no good with nail heads and mud. Waterproof hiking boots or grippy trail runners are great. Sneakers are okay but slippery. Expect your shoes to get muddy. No matter what you wear, if there's enough rain, it's going to end up inside your shoes.

Pants: I know everyone loves their jeans, but soggy jeans are not fun. (If you're planning a backpacking tour of Japan, I suggest a pair of fast-drying synthetic pants that take up half the volume and weight as a pair of jeans, and if you wash them at night, they'll be dry (or nearly dry) in the morning.)

Shirt: Synthetics dry faster than cotton.

Extra Clothes: Essential in cooler months, an extra fleece or a change of clothes is also nice for the ride back after a rainy hike.

Optional/Seasonal:
Gloves: Don't expect them to keep your hands dry, but many folks hike with gloves for comfort and/or warmth.

Hat: To keep the rain out of your eye, and also to keep warm in cooler months.

Safety
First Aid Kit: Everyone has their own style. Bandaids, gauze, medications, and tape are a good place to start.  If you hire a guide, of course, your guide should be carrying a kit.

Whistle: Especially if you are hiking on your own.

Optional:
Compass: If you get lost, I recommend staying where you are (or going uphill to try and make cell phone contact in some cases) and blowing your whistle, but there are often stories of folks who bump their heads, get turned around, and head the wrong way, or just make a Marker Tape:wrong turn at the trail head.

Marker Tape: This is for emergencies only. If you get lost and resort to using marker tape, please tell someone so that it can be removed lest others follow your mistaken footsteps.