I will be the dissenter here and just point out some of the potential negatives of taking a rabbit "for a walk." The personality of the individual rabbit plays a major role on whether or not this should even be considered.
Those who do agility with rabbits are the experts on what types of harnesses are safe (most types sold at stores are quite dangerous) and on how to properly fit them. A 'safe' harness that is not fitted properly can be just as dangerous. The very real dangers of an improper harness (or improperly fitted) are a broken neck or back.
Assuming one has a rabbit willing to be harnessed, and assuming one has the proper harness, and assuming one has fitted it properly, the next question is where to take the rabbit. If you plan to use the harness to take him out in your own yard, fine. It is a place with which he can become familiar. It is also a place where you know that the grass is safe -- no fertilizer, no weed killer, no pesticides.
If you plan to take him about the neighborhood or to a public park then you'll have other issues to deal with. The safety of the grass itself (as mentioned) is always a concern. You won't know what lawns have edible grass. These other grasses may also have ticks, fleas, or other parasites. In addition, there is the danger of your rabbit suddenly bolting at the sight of a dog, or kids, or a car, or a car's horn, or children screaming while playing, or people approaching, or an overhead shadow, or unfamiliar smells and sights. Any or all of these may seriously frighten certain rabbits. If they bolt in reaction, serious injury can be caused by the restraint. If they don't bolt, they still may become severely stressed.
Many rabbits prefer familiarity but going to strange or new places is a stressful and frightening experience. Going on such outtings may be an unpleasant experience for a rabbit.
Of course there are exceptions, and some outgoing rabbits seem fine with the unfamiliar. Some are naturally bold and fearless.
There is always the option of using a pet stroller for those owners who desire to show off their rabbits in public (who doesn't?)
. I've used a pet stroller for my rabbits in the past.
If the motive behind harnessing is simply to let bunny be outside in fresh air, then it is probably easier (and more enjoyable for the rabbit) to be in an exercise pen in your backyard. This method allows bunny to move freely without the constraints of a harness and leash. Of course, it is necessary for you to be outside with him to keep him safe (your presence will deter birds of prey). But he can move about within the pen and safe from access to any toxic bushes. He can also become used to the same surroundings making the backyard a more familiar (ie. less scary) place.
If the motive is exercise, skip it. Aside from those doing official agility stuff, the bulk of harnessed rabbits don't happily hop down the sidewalk like a dog. They spend much of their time in one spot grazing, take a few hops, and graze again. It is more of a bore for you. Indoor rabbits get their best exercise when free roaming indoors where they feel safe. That's where you'll see their bunny 500s and binkies. They cannot run full speed while confined by a harness and they cannot binky too freely either. To do those fun dashes and speed runs, they need to be free of a harness and leash.
So there's a list of some of potential downsides for consideration. I'm not saying it should not or cannot be done since clearly it can work fine for some people and some rabbits. It's just good to be aware of those downsides as well.