We just returned from a May departure. Here is some packing advice based on our experience.
1. Men do not need to bring a suit jacket. For the welcome and farewell dinners men wore button down shirts or polos with neat slacks. Women wore casual dresses or slacks and tops.
2. You do not need to change for dinner most nights, but there are some nights you might want to. The same type of clothes you wear to the welcome and farewell dinners are fine for dinner in the hotels.
3. Jeans and shorts are perfectly acceptable. If we were in doubt about what was appropriate, we just asked the tour director the night before.
4. You do not need to wear white socks in the temples or shrines. Any socks are fine.
5. There are a number of times when you will take off your shoes so a good pair of comfortable slip-on shoes is best. There is some seating everywhere you need to take your shoes off so you can sit down to tie your shoes if needed.
6. All of the hotels provide pajamas or yukatas but they are not sized for larger people. My guess would be the largest pajamas are about a US women's 12/14. The yukatas are similar to robes so the sizing is more flexible. If you don't think the sizing, or sleeping in a yukata is for you, bring your own pajamas.
7. For the night at the onsen, when everyone wears the yukata to dinner, larger people wore pants and a shirt underneath the yukata. The yukatas are navy and white so if you want to color coordinate, pack a white or navy t-shirt.
8. Don't bother packing a bathing suit unless you are adding extra time to your stay. This is a very busy schedule and there isn't much time for swimming. In the hotel and the onsen that have the mineral baths, you bathe naked. Don't be afraid to try it. No one looks at you, and you'll never see them again anyway.
9. In May, a light, waterproof jacket was perfect. Be prepared for some rain along the way. The hotels have umbrellas you can borrow.
10. Moisture wicking shirts and breathable fabrics are best as it was hot some days. Travel clothes from Columbia, Eddie Bauer, REI, are perfect for this.
11. Laundry service is available about halfway through at the hotel in Kanazawa, but at hotel prices. $$$
12. All of the hotels provide shampoo, conditioner, soaps, hairdryers, disposable toothbrushes, toothpaste, etc. You can use those or pack your own depending on your preference.
13. There are two times when you will need an overnight bag so bring a small bag that you can carry with you on the trains to take only what you need for just one night.
14. Finally, leave extra room in your suitcase for all the fun things you buy during the trip!
