I've added some updates to this because of products failing. Because they failed for me does not necessarily mean they will fail for you. I will show you which one's I have had problems with and remove them when I can find a replacement.
So you'd like to travel light with one bag eh? Well, you can spend hours reading (recommended) the many interesting web-sites such as onebag.com or Randy Johnson's Travel Page, that explain how to do this, or you can save yourself a bunch of time and check out what I have found to work. Even though some of the products that I mention are for males only, females can also use this guide and just search out similar products from the same brands mentioned.
First your going to need a Packsack that will fit the FAA's guidelines as a carry-on, which are 22" X 14" X 9". These guidelines are not necessarily followed by all airlines, some allowing more, some allowing slightly less, such as 55cm X 35cm X 25cm. There is only one decent back-pack that is front loading and for men that is the Kelty Redwing 44 Internal Frame Pack (Kelty Redwing 2650 Backpack older model) or for woman the Kelty Women's Redwing 40 Internal Frame Pack (Kelty Women's Redwing 2500 Backpack older model). These packs dimensions are rated slightly over the allowable limit, but there is nothing stopping you from not having these packs stuffed to their maximum sizes, thereby keeping them down to the allowed dimensional limits. There are only a couple of other packs that are similar to the Kelty pack, but they do not have waist belts. I have owned and used my Kelty Redwing 2650 for the past 15 years in the bush prospecting and have also used it as a carry-on bag. Very durable and comfortable. Another option is to get a custom built pack from either Ostrom Outdoors or McHale Packs. These offer the ultimate in durability and fit, but can be quite pricey. To keep your Redwing Packsack and it`s contents dry you will need a small Sea to Summit Deluxe Pack Cover.
Next your going to need to find a way to compact everything down so it all fits in your Packsack. This can be done with a Sea to Summit eVAC Dry Sack. You will want one for your Shirts and Pants. One for your socks and shorts. And one for any additional clothing such as a fleece jacket, underwear, gloves and rain gear. You will also need one for your Laundry.
Now on to what to pack in those compression sacks. For the ultimate in comfort and usability the ExOfficio Men's Air Strip Lite Long Sleeve Shirt. This shirt comes in regular or tall sizes, with long or short sleeves. For pants I would recommend a pair of RailRiders X-Treme Adventure Pants, and also a pair of their Rampage Shorts for the beach. For men`s underwear the ExOfficio Men's GiveNGo Brief - 1241-0008 is worthwhile trying out. As for socks, everyone has different shaped feet so you should try before you buy. So to add things up, all one needs to carry in their Pack is 1 pair of pants, 2 Shirts, 3 underwear and 3 pairs of socks. Laundry can be done in the sink every few days, which we will discuss below.
For lightweight hiking boots there's the Hanwag's Banks GTX boots (came apart within one week of use, but replacement had no problems) and for lightweight packable shoes there's Merrell`s Men's Barefoot Trail Glove shoes. And for those filthy showers or for just puttering around the hostel with, you can buy a pair of Ida`s Ultra Soles.
How does one keep one's self and their laundry clean? I would just use your favorite shampoo, preferably something that is biodegradable and non-scented. To dry your clothes there's the
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and Travelon Set of 2 Inflatable Hangers. Let's not forget to bring a Do it Rubber Sink Stopper being that they don't exist in lots of third world countries. Lunatec makes a nice synthetic washcloth, which is quick drying and odor resistant. And the Sea to Summit DryLite Towel can be used for drying yourself off, drying your cloths off or for soaking up rays at the beach.
For carrying on liquids such as shampoo and conditioner you will need a 3-1-1 compliant bag. A Carry-on Compliant Frequent Flyer Kit will come with everything you need. You can easily place your conditioner, shampoo, toothpaste, skin lotion, suntan lotion in the squeezable/collapsable plastic containers they supply (Ends failed on two of these bottles).
Chums makes a huge variety of eyewear retainers. You wouldn't want to loose those $200 Maui Jim`s now would you? And to keep your cap from blowing away there is also a large variety of good Cap-Hat retainers.
Don't forget to bring a camera to capture images of all those amazing sites. A Pentax Optio W90 12.1 MP Waterproof Digital Camera with 5x Wide Angle Zoom and 2.7-Inch LCD (Black) is a very durable water-proof camera. After having read all the reviews on all the water-proof compact cameras, one finds that they are all poorly rated as being actually water-proof and do seem to end up leaking after a short while for most. You can carry a larger SLR camera with multiple lenses which will take amazing pictures, but if it's too large to lug around with you everywhere you go, you will end up missing great photo opportunities. You should have a camera at your side at all times. This one is small, durable and also takes videos. Of course if you have a great photo opportunity and don't have a nice DSLR camera with you, then you will probably be disappointed with your point and shoot images and videos. For better videos there a huge choice of camcorders out there, which can also work as a digital still camera, and a good DSLR can now shoot amazing video. Don't forget to bring a Joby GP1-A1EN Gorillapod Flexible Tripod (Grey) so your not always having to find someone to take a photo of yourself.
When going on a day-trip you will need something to carry your camera, camcorder, toilet paper and rain-jacket/pants. A Pacsafe Stashsafe 100 Anti-Theft Hip Pack in Brown should do the trick. It's size and the colour makes you not so obvious. That said, you are going to look like a tourist with one of these on.