Many carriers come with extras. I generally eschew the add-ons as they unnecessarily increase the carrier’s weight. The most any person can carry on their back about 60 pounds, and beginning hikers probably won’t be able to haul much more than 30-35 pounds, which is about the weight of a toddler and their carrier.
Still, sun and rain guards as well as canopies are popular, even though they generally don’t provide the perfect sun and wind protection as claimed, for at just the right angle (which almost always seems to be the angle the trail takes), the elements still can get in. Children usually don’t like looking through plastic windows, which often become scratched and discolored after a few hikes. In addition, they can be awkward to assemble and can’t be easily stowed on the trail if you want to take them on and off. A simple hat with a wide brim and properly layered clothing will keep sun and wind off your child while giving an unhindered view. In some regions, like the Pacific Northwest, hiking without risk of rain may be impossible for long stretches of time, so you’ll have to decide if the benefits of this add-on outweigh the negatives.
Another add-on is stirrups. Feet can dangle, and though kids may prefer that, stirrups are helpful once they start kicking about. Kids in baby carriers do swing their legs, and their feet can hit you hard in the back or ribs.
Places to strap things on and extra pockets are fun, but I prefer a fanny pack. To reach anything on a baby carrier, you’ll have to take it off, and that is inconvenient when hiking with children.
Read more about day hiking with children in my guidebook Hikes with Tykes.