A few of my favorite resources included:
- NCTM's Student Math Notes (now called Student Explorations in Math) These are copyright protected but available with a small membership fee.
- Cynthia Lanius' website has fun math explorations suitable for class or clubs.
- For my elementary students we worked various puzzles - like these or these!
- We worked sample contest problems - samples at Math Olympiad
- And we played favorite games like Set, 24, and Equate! There is a SET tutorial and daily game online. There is a 24 game online as well!
- We read Theoni Pappas' stories about Penrose, the mathematical cat!
I haven't sponsored a math club for several years. Instead I sponsored a writing club ... which was different and fun! If I were in an elementary school this year I would consider combining the two ideas ... math stories, math poems, and math problems!