This will allow them to see up to several words at a glance while reading. They will then learn visualizing techniques to help them to gain increased comprehension at faster reading rates. Anything that is at or below their grade level is fine. A watch with a second hand. A pen to tap with.
Most people see the whole tree in one or two glances.
This is what we call Natural Vision. When we are seeing almost anything, we are using Natural Vision, except when we are looking at a page of print. You see, in first grade we were taught how to have tunnel vision. Most people can only see one word at a time. You may as well read through a straw.