What to definitely pack on RW trip...

rubber door stop for intruder protection

Duct Tape - repairs everything stick to everything
bungee cords for in room clothes hanging
big plastic garbage bags

Agree with most of the above. Bungee cords sounds fun! and never heard of the rubber door stop but sounds quite clever.

some very good suggestions. My advice is a very minumum of clothing, only enough to wear and cover up while washing your every day wear, and a good set of silk long johns if any chance of encountering cooler climates. Socks, gloves, scarf and top and bottom will wad up about the size of a tee shirt. I also like a silk bed sheet body bag. Almost anything else can be bought along the way. Everything fitting in one smallish convertable soft bag-backpack will really ease travels.

My secret travel item is diaper pins. They are excellent for security - I use them to close backpack zippers, pin pockets closed, and pin my fabric wallet pouch inside my pants. They are tricky for a pickpocket to open quickly, and won't stab me in the belly by accident. One can always use extra safety pins anyway. I usually start a trip with pins carefully hidden away, but arrive home with them stuck all over my clothes. They are getting hard to find now, so I have stocked up with several packages.

On a RW trip, I'd take a dozen, at least.

I find the twisted elastic clothes lines invaluable, as well as the lightest weight wire coat hanger you can find - just one - It takes up no room when it lays flat in the bottom of your pack. It can be straightened out in an emergency to make all sorts of things - I used mine once for hanging wet sneakers that refused to dry inside, outside of a 3rd floor window in the breeze.

Washing line of some sort - you can buy twisted bits of elastic with a couple of hooks on the end that are more sophisticated and more flexible that the pieces of string that i used in the past. You can hang the clothes through the twists.

Power adapters are expensive in Western countries. In asia you can by them for less than $1 so it depends where she is travelling.

Superabsorbent towel.

A couple of ziplock bags. Washing powder can be bought in small quantities in many countries but it is useful to be able to contain it in a plastic bag so it doesnt get into every thing else.

one big strong garbage bag to line the suitcase/pack when travelling in wet conditions.

Tampons - there are many places that you cant get them

I think you shouldn't take everything that has been recommended because of weight. There are some good suggestions, here, but don't try to take EVERYTHING that you think you COULD need. I second the suggestion of the minimum of clothes---I take (or wear) 3 tees, 2 pr of pants, 3 sets of undies, 3 socks, sandals, shoes, a swim suit, a zip up sweatshirt and a shell jacket. I don't need earplugs, or a pillow, etc. One can learn to sleep through noise---how many times have I fallen asleep at home with the TV going? Whatever you take, you'll have fun, no doubt! Oh, and the towel---not a big bathtowel, but a little 'travel towel' which is about 12" X 16"---that's big enough and it works.

Apart from most of the above, I usually take a small immersion coil (dual voltage) for boiling water in places were the water is dodgy. That could mean carrying a small cup as well, but it came in handy in India and Africa when there was no drinkable water available. Also good for making morning tea &/or coffee.

I'd follow Lor's #18 recommendations.
People tend to pack too much in their luggage and half of the stuff they carry turns out to be obsolete dead weight. It's not possible to foresee all kinds of situations on the road. Flexibility and resourcefulness are the key.
Apart from clothes, the only three things I take with me are: camera, mobile phone, socket adapter.
One piece of luggage only is very practical, not more than 35 kg (although I'm aware for many that would be too small).
From what you're saying, I understand, your niece is going to travel in warm climatic zones, so she'll need light clothes only.