During guided reading, I always have at least one student that finishes "whisper reading" before the others. Usually I have them read the story again for fluency, but sometimes I give them another task. I have them search for a certain spelling pattern in the story. For example, if we are studying long i, they would search the story for long i words and write them in their notebooks. This doesn't take any extra copies or work on my part, but it's something for them to do while they are waiting, and it gives them extra spelling practice. To make it a little more fun for them, I give them a pipe cleaner curled up like a "magnifying glass" to help them search for words.
Here's a picture of a student looking for words using a pipe cleaner after we read a poem as a class.
Other times, I'll give early finishers a reading response question to answer in their notebooks.