As you read, you'll hear me refer to rides our daughter takes in her backpack. While this is a pretty commonplace piece of baby gear, I thought is worthwhile to offer a few suggestions about this since it is essential to the dayhiking parent of an infant or toddler.
Safety is the primary concern, so look for a backpack with an adequate harness. Most only have 3-point systems. This has worked out fine for us. Be sure that the carrier you buy will accommodate your child's weight. There seem to be two cutoffs, 35 lbs and 48.5 lbs. If you do a lot of moderate to difficult hiking, I recommend getting a carrier with a higher weight rating. That way, you can tote Jr. around until he's almost 3. Also, don't put your baby into a backpack carrier until he's able to hold his head up confidently. Smaller babies belong in a front carrier. Refer to and follow the manufacturer's safety guidelines.
We have a model manufactured for Graco that has an injection-molded plastic frame and nylon body. It is equipped with a 3-point safety harness and removable carry-all. When Eliana was smaller this was a great piece of gear. As she has grown, however, it is proving to be less than ideal. First, it is getting uncomfortable for me. The shoulder straps aren't sufficiently padded, so that as my daughter gets heavier, the straps dig into my shoulders. And although the pack is rated for a child up to 48 lbs, it is uncomfortable to use for extended hikes with Elle at 30 lbs. Also, the bottle-holders built on the outside of the carry-all are too small for a sippy cup. But the most disturbing issue is that the safety harness is too getting too small. This is unexpected since she could gain another 18 lbs and still fit into the weight rating (and my daughter isn't especially tall). I can't complain too much, we've gotten plenty of use out of it...and we bought it at a consignment shop for $30.
From my own research (since I've got to replace the one I have really soon), I've concluded that it's better to buy a baby carrier made by an outdoor outfitter instead of one made by a baby gear manufacturer. Baby gear manufacturers don't really have the dayhiking enthusiast in mind. An outdoor outfitter is more likely to think about details like shoulder strap construction and overall weight (the model we have weighs about 11 lbs empty). They make packs that real backpackers use to carry 60 or 70 lbs, all day over strenuous terrain. There are a number of websites that sell backpack carriers, just make sure that you get one that will accommodate your baby for as long as you intend to use it. The max weight tends to be 48.5 lbs but some are as low as 33 lbs. I'll come back with a recommendation once I get a new one.
I'm looking to spend about $150 for a new piece of gear that I'll only get about a year's worth of use from...but there's always Ebay.
Have fun!