The main thing you want to do is treat it with reverence and respect.
If someone were distributing communion and dropped the Sacred Host, normally he would pick it up and consume it. So if you would feel comfortable consuming the host, that would be one possibility.
If you don’t want to consume it, the other thing that can be done is to allow it to dissolve in water so that it is no longer bread, then pour the water into the ground, perhaps in a flower bed where it won’t be walked on.
Even if a priest isn’t around at the time, I’d let him know afterward that you had found a host. It’s something he should be aware of so that it can (hopefully) be avoided in the future.