Hi, Corinne,
I admit a twinge of envy. I now teach high school math, but I miss the
days of middle school math. You have a great opportunity to engage and
develop your students minds when they are open and eager.
I highly recommend joining Mathcounts Club.
http://mathcounts.org/club
They have 5 or 6 short monthly competitions, but they also have
wonderfully engaging lesson plans and even some supplies. It's free to
join, and if you do, you will gain access to past archives with more
ideas. If you have really strong students, you could look further into
the competition level, but that wouldnt be necessary.
A similar program is Math Olympiads
moems.org.
It costs $100 or so to register. Their flagship book, Creative
Problem-Solving for Schools, has a variety of excellent lessons.
I know competition isn't the best fit for every student, but the MathCounts Club, especially, is pretty low key with lots of non-competitive activities. There is also something to be said in defense of competition. For some students, it is very motivating. For most, it gives a reason for learning the concepts so that they can apply them in less routine ways. Competitions have helped my students to learn how and when to use their math tools. It's given them a great foundation for high school.
Godspeed, Sandy Hollenbeckk
Godspeed!