If it wasn’t for the shopping I would say to take the smallest pack possible. A good pair of sandals, a week’s worth of underwear, toiletries, one set of comfortable longs (for flights and those pesky air-conditioned buses), one pair of shorts for the rest of the time, swimwear, sunglasses, hat, and a T-shirt from Mozambique or somewhere similar. You don’t want people to think that this is your first trip anywhere. 
You’ll pick up more clothes along the way, so no need to take more. Ditto the sarongs that will double as sleeing bags and beach towels. Never ever travel with a pair of jeans.
This much all packs up really small, and you can probably go hand-luggage only when you fly.
Then there’s the other stuff. Music, for instance. I remember being so pleased when I dumped all my tapes and bought one of those new-fangled CD walkman things. There are times when you really need to drown out the world around you and listen to your fave tunes, so you end up lugging a big box of music around. Nowadays you can go MP3, which is just marvellous.
I thought of getting an iPod-style thing, but my PDA plays music quite satisfactorily. It uses flash memory sticks, so you’re a bit limited for choice, but a 512Mb stick is good for 5-6 hours. A few of those will give you enough variety to get you through.
The other advantage of the PDA is that you can load it up with eBooks, if that’s your thing. Books are HEAVY. You can even make jpgs of maps and documents out of any info you find online before you go so that you have an electronic guide book.
Cameras. I had (just left it in a taxi) a digital, which meant that I had to carry a charger and make sure that it had enough memory to store all my pics for the whole trip. Actually, I often take my laptop with me so I can move my pics, write my emails, and play my music. Spare set of speakers makes all the difference. It’s amazingly cool to go back to your digs, fire up some loud rock while you shower, view your pics full-size, and write up your day’s adventures. Then you put whatever you need onto your memory stick and pop into the internet cafe to send it all and pick up any incoming.
It starts to sound like a lot of gear, but actually it will all go into a hand-luggage size bag. It sounds silly, but I stoppped using a backpack too. People laugh when you walk down the street with a wheely-case trailing behind you, but I’m not the one with 20 kilos on his sweaty back so fuck 'em. You can throw a soft daybag into your case so you have a lot more versatility. If you’re trekking then why not leave the stuff you don’t need in storage in one bag, and carry the other with your essentials?
You can carry a PDA, camera, money, sunscreen, and water bottle in a bum-bag, if it has enough pouches, and throw your sarong over your shoulder (or wear it) if you’re going to the beach. No need to carry a bag at all. Put the weight on your hips, not your shoulders. When you travel you have your luggage roll along behind you instead of having it weigh you down.
I’m serious, it’s the best way to go, until you find yourself walking down the beach in search of digs. How much actual walking do you do on surfaces where wheels don’t work? It’s only when you go trekking that you really need a backpack, and then you don’t want to take everything anyway. You’re going to leave a bag somewhere, why shouldn’t it have a padlock on it?
In conclusion, your luggage should consist of:
- a bumbag containing your valuables
- a wheely case small enough to take on an aeroplane as hand luggage
- a soft backpack for shopping or trekking, carried inside the wheely case
Have fun.