Female edition - summer version
If you are traveling in Southeast Aisa, you should use backpacks in the following countries: Indonesia, Sri Lanka and Cambodia. In Malaysia, Thailand and Singapore, it is not mandatory. I haven't been to the rest of Southeast Asia.
It is neat to pack light and in hand luggage for several reasons, and one of them is - you will not be paying additional money when flying locally in Southeast Asia, and many times it is more practical, sometimes even cheaper to fly than to travel any other way.
Because of humidity and organizationatal purposes, pack your things in plastic bags. You can roll your clothes to make more space in a backpack, but you don't need additional space because you want to pack light, so I do not think rolling is necessary, unless your main backpack is really small.
Bigger backpack which fits hand luggage dimensions:
Bag #1
underwear
Bag #2
bras
Bag #3
8 dresses
Bag #4
5 shirts, 2 cotton scarves
Bag #5
3 shorts
Bag#6
Shampoo, shower bath, tester perfumes, mosquito repellant, sunblock creme, facial wash
Bag#7
Pills
Bag#8
2 bikinis, swimming goggles, scarf for beach, cotton hat
On the sides:
Plastic shoes
Black sandals
Golden sandals
Book
Purse
Smaller backpack, or a bigger purse:
Bag#9
Toothpaste, toothbrush, jewelry, q-tips, cotton pads
Bag#12
Facial creams in small packages, soap
Bag#11
Makeup, padlock, tweezers
Bag#12
Camera and phone chargers
Bag#13
Small towel, 3 pairs of socks
Also:
Wallet
Passport
Phone
Sunglasses
Ear plugs, sleeping mask
Camera
Wear:
Pair of jeans
Cardigan
Shirt
Sports shoes
If it is raining, you can buy very cheap low quality rain coats in supermarkets that will serve their purpose during your stay.