same subject...
I strongly feel that students should do something in the same subject area that we're working on. So, if we're doing a math activity and they finish early, they should choose an activity from the math center. If we're doing writing workshop and they finish (which they don't because it's a workshop aproach, and they're always cycling through new projects), but if they did they shoud do other writing. Same goes for reading, science, etc. So in the perfect world, there would always be something related to the activity for them to do when they finish. Of course that doesn't always happen, so in my room when need be they can always read/look at books when they finish their work.