Except I want them to be able to do more than just interact with other spirits. I was thinking some kind of magic.
Let me put my expository venturing hat on. Ahem.
Spirits are naturally chaotic, it's what defines them. In contacting our world, they bring chaos with them wherever they may go. This usually manifests when they overlap with denser areas of matter (while their movement through air can produce physical aberrations (both material, like a sudden skyrocketing of the neon content, and otherwise, such as decreases in temperature or a change in optical properties), this is less noticeable than most of their other activity). Once spirits overlap with certain material, they can start to alter it slowly, considerably slower than they could, say, a human being, but it's also less likely to introduce structural changes in the spirits themselves. What they can do just as quickly as with a human being is randomize a given object of their volume, letting loose their chaotic natures. This usually results in a loss of function for the object in question. Naturally, a spirit can only overlap an object for either purpose for so long - structure starts to be imposed upon it just as much as it passively would be in a human body.
On the possession front, spirits can also possess animals other than humans, and even other lifeforms, though with animals it seems to be easiest to get the desired rates of alteration. They can, for instance, easily possess a cat with little to no worries that something might happen to them (since the cat has hardly the presence of mind to question the spirit's presence, and it is impossible for it to be learned in the methods of destroying a spirit, unlike a human being). On the other hand, they can't control the cat's mind in any way and, lacking in intelligence as the average animal is (to say nothing of the average plant, fungus or prokaryote), they can seldom further their own objectives effectively with these alterations. It does work wonderfully for all sorts of mischief and distractions, though. Some spirits have, in the vein of humans training their domestic animals, tried to do the same, with some success, even. Corvids especially seem like popular targets for spirit-directed training, hence why people seem to have an even greater distrust for them near the pathway than usual.