The site mentioned in Greg's answer, Art of Problem Solving, was my old haunt in high school (the pre-MO days!) so perhaps I should say a word about it. First, there is a large forum there with lots of people posting interesting questions, and you might recommend your students to browse and perhaps join; that was my main source of practice at doing mathematics in high school. Second, there is a Resources page with previous questions from many famous Olympiad-style competitions; I'm really only familiar with the US competitions and the IMO, but of those I can suggest you look at USAMTS and AMC 10 questions for interesting problem sets.

Of course you are not training these kids for an Olympiad so the focus on Olympiad material shouldn't go too far. Nevertheless, I agree with Greg that doing mathematics is much more engaging than listening to it, so I think you should be devoting a decent amount of your time to finding interesting problems for them to do, structured around certain general topics if possible. The Art of Problem Solving books, although geared somewhat to US students preparing for competitions, represent what I think is a pretty good selection of interesting but elementary material.

(This is all assuming they aren't already hardened Olympiad veterans.)