This is a common problem with learners of this age. There are two ways of tackling it: one is as you suggest to have fillers to keep the students occupied. The other is to re-think how you organise the work so that the quicker students have something more challenging to do.
You could pair the stronger students up with weaker students to provide help and guidance. Make sure this is the case so the stronger students don't just "take over". You'll know your students well enough to see whether this is an option.
Alternatively, you could give the stronger students a more challenging task. For example, with a reading activity the stronger ones could have more difficult questions or simply more questions to answer. There are two schools of thought on this one. Some people think that organising work in this way "shows up" the weaker students. Others think that everybody is aware of the distinction and making it public makes it easier for everybody to live with. Think about where you stand on this one and be sure to handle it sensitively in class.
Some students who finish quickly are not very careful with their work. If this is the case with your students, you could try encouraging them to be more careful by going back and reveiwing their work.
If you decide to use fillers rather than (or as well as) re-organising the work, you can keep a stock of photocopiable puzzles in a corner of the classroom. Students will soon become autonomous and will work through the puzzles themselves. Students can also use this time to create simple puzzles for each other (this will also save time for you as you'll just have to photocopy them, rather than look for puzzles yourself). You could simply allow the early-finishers to do their homework in class, which will give them more free time at home. Whatever you decide to do, it should be enjoyable. Students shouldn't feel punished for finishing early.