In my experience there are three kinds of students - ones that finish an activity on time (yay!), ones that you have to push to finish an activity on time (boo!), and the ones that finish early (YIKES!).
I have been helping a friend who just started notebooking and teaching fourth grade science and those early finishers have been problematic (mainly because they start bugging everyone else who is still working :)
We recently devised the "poster project" for students who are done early. Basically we set aside a part of her wall for these posters (in this case her cabinets as pictured above). Students who are done their assignment early are asked to contribute a poster to the wall. It has to be subject specific (so no random pictures!). The other day she did weather tools and the kids who finished early had to draw and label a poster of their favorite weather tool (she uses regular paper and the kids have to mount it on colored construction paper and put it up on the wall....which they LOVE to do). She was working on weather safety the other day and if they were done their notebook assignment they had to make, mount and post a weather safety poster.
I've been grooving on this idea to the point I think we need to set up a poster supply area for these children so that crayons, markers, construction paper, glue, and tape for the wall are all in one "ready to go" to area. We only get at most five to seven children who might be done early (and they don't necessarily have time to finish their poster - if they are not finished we keep them in an "unfinished poster" file and if they are done something else early they can go back to it.)
If you are short of wall space you could always put the posters out in the hall under "What We Are Studying in Science" header.
Another reason why I like this idea is under our observation rubric we must have student work on display. This activity pretty much guarantees that we have met that objective (another YAY!).