These are a bit specialised. The lighter the gear, the farther you travel, but the shorter the life if you mistreat it. While a conventional internal frame overnight pack can weigh 2.5 kg quite easily, full-size ultra-lightweight frame packs come in closer to 800 gm. Good ones work very well. The author makes his own and uses one all the time. Apart from saving about 2 kg in weight, it is more comfortable.
You will also see a fair bit about even lighter packs in America, down to a few hundred grams. Some of them look very glamorous, but several words of caution are required here. First of all, these American UL packs have no frame at all, and are only meant to carry very light loads. They come out looking like round balls when full, and their ergonomics are terrible with significant loads. Actually, their ergonomics are not very good anyhow, but you don't notice that if the load is light enough. The second problem lies in the materials and construction needed to get these very light weights. The fabrics are extremely light and can be shredded in the Australian scrub. (They can be be shredded in American scrub too.) In addition, the stitching can't be very strong when the fabrics are so light, and it has been known for the shoulder straps to tear out and disintegrate under load. These packs are meant for fanatic ultra-lighters on short trips, and for 'adventure racers'. They are good for their intended applications.
These are a bit different from other packs; They are meant to be thrown in the river and used as floats. They must be waterproof! They get bashed against canyon walls and dragged up and down them, so the fabric and design must be smooth, without bits to catch on things. This means they are usually made of fairly heavy-grade welded PVC-coated (or PU-coated) fabric with a special roll-down seal at the top, and no other external fittings at all which might catch on any rock. Well, pity about the shoulder straps. They are not cheap, but can make the difference between a dry lunch and jumper and a completely soggy mess. On the other hand, they are seldom as comfortable to carry with any large load in them, although I sometimes think this is as much due to the shape of the loads they have to carry. For this category you should examine other peoples' packs and get specialised advice.
A very common substitute, which is highly recommended, is to buy a waterproof canyon liner or ""dry bag"" (without shoulder straps). Again, the dry bag is made of medium-to-heavy PVC fabric with a roll-down seal: it's just a bit lighter. You put it inside your ordinary day pack (or overnight pack for really serious stuff) with all the ""dry"" things inside. It helps if it is just slightly larger than the day pack, so the fabric of the dry bag is not stretched. This means your day pack is going to get bashed around a bit, but that's what happens in rivers and canyons. More recently, people have been using silnylon 'dry bags inside their day-packs. The seams need to be well sealed, but the result is very light. You just have to be a little more careful to not pack sharp objects against the dry-bag wall.
Incidentally, when you open a dry bag in a canyon your hands will be dripping wet, and this water will get all over your precious dry stuff. Stick a bit of an old towel right at the top of everything inside the dry bag to dry your hands first. Very useful for when you want to get your camera out quickly.
Climbing and Alpine packs
These are getting very specialised. Don't buy one until you have had some experience and seen a few in real use. They differ fundamentally from a day pack or an overnight pack in that they try (correctly) to put the weight as low on your body as possible. This is great for real climbing, but the opposite of what you want for almost everything else. They are also usually a bit small for lengthy trips. Another disadvantage of a climbing pack follows from the shape: it is narrow at the top and wide at the base, making it very hard to get things into it or out of it.
Hostel or Travel Packs
A very important difference you must watch out for is that between ""real"" bushwalking packs and hostelling or trekking packs. A hostelling pack is designed to be opened a bit like a suitcase; a bushwalking pack is more like a large bag. A hostelling pack often has a large zip right over the top: lay the pack down, unzip it right around, and behold: all your goodies spread out. Well, fine, on a hostel bed. Granted, your clothing may be packed more elegantly flat in a hostelling pack than stuffed into a bushwalking sack, but that's life.
The zip over the top is a disaster in the bush. It can rip, snag, open when it shouldn't, and let water pour in when it rains - even with the little protective baffles the manufacturers put over them. My experience with such zips on packs is not good. I once lost an expensive SLR lens out of a pack with an over-the-top zip, in thick scrub below Hat Hill Ck near Blue Gum Forest. The zipper slide caught on a bush and it opened up. Never again. (It is in an orange nylon bag, should you ever find it.) A bushwalking pack is a sack with a throat which can be done up, and has a lid which comes down over the top to protect everything underneath it. Of course, little pockets on the outside with little zips where the zip is fully shielded are usually OK, as long as they aren't very big and don't stick out at the sides.
You can take small babies walking - with care. We have to consider what a baby needs to decide what is suitable: two main forms of baby carrier result: the sling and the papoose. The main distinction is whether the baby is old enough to hold his/her (it's?) head up without assistance. Below a certain age you have to support the baby's head, so the fabric sling carrier is probably the best. This also means the carrier (usually mum?) can keep a really close eye on bubs. Above that age the baby can sit up and look around, so a ""papoose"" carrier is suitable. Even so, baby is going to need some head support some of the time. They sleep quite nicely on your back.
The sling carrier is a diagonal sash with a pocket. The baby goes in it, between the sash and the carrier, so the sash holds the baby and its head up all the time. It looks a little ""third world"", but is extremely effective. Babies seem to be very happy in these: they are after all snuggled up next to Mum! They can be bought in baby shops (here I pass). Fortunately, babies don't stay this size for very long.
The papoose is a frame with a bucket seat, is carried like a pack, and the baby sits in it looking forward past your head. Rather than try to describe it in detail we suggest you look at one in a shop. Better still, find someone who has one in use. Things to look for are a reasonable range of size adjustment (babies grow fast), very good soft padding around the top rim, and some way keeping the pack upright when you put it on the ground. However, if the padding around the rim is not enough (it often isn't) you can always add some soft foam yourself. Baby will go to sleep leaning on that padding: make it soft. One word of caution: never ever give a child in a papoose anything sticky. The stickiness gets on their hands, and then they start to play with your hair
Do not think you have to use an internal frame pack! There are many other sorts. As mentioned at the start, external frame packs used to be popular in Australia but these days they seem to be restricted to America. They are well known as being good for for long trips and large loads. And if they work well for those trips, they should work even better for short overnight trips. However, finding a suitable one here in Australia is a bit hard these days. This is getting technical, so you will need to look in some American catalogues to see what is available. Alternately, go light-weight.
Some packs have no frame at all: they have a fixed harness. Most day packs are like this. It can be more difficult to get a good fit, but good fixed harness packs come in a range of sizes for that reason. They have the advantage that the harness is much smaller, lighter and cheaper. The sheer bulk of an adjustable harness can be a problem in itself, while a good fixed harness pack, properly padded down the back, can be much smoother. If you can go really ultra-lightweight, a ""frameless"" pack might be just the thing. Be careful though: part of the reason these ultra-lightweight frameless packs seem to work is really because the user is carrying so little weight. Load them up and it is a different story. However, it's fair to say that choosing a large fixed-harness pack for long trips requires a bit of experience.
You could go back to the swag: contributions from anyone who has tried it would be appreciated.