make an effort to speak with them a little before and/or after class. ask them a couple of questions that befit their level.
I do this with my good students. They seem to appreciate it.
I also offer optional assignments for the kids who really want to do more. I work at a "key" high school with extra super motivated students and the good ones actually do the extra optional assignments.
Mixed level classes are a real pain in the ass though, especially if you end up with some kids that are very advanced mixed in with those who can hardly throw a sentence together.