John McBildit's Staircase Problem is one of my favorite math projects for back to school time. It takes just 2-3 days, and is a great intro to the variety of ways to approach math problems and explain mathematical thinking. I have used this successfully with second and third graders.
I've spent 12 of my 14 years of teaching in third grade. I create easy to use, classroom tested resources for 2nd and 3rd grade teachers. When I'm not thinking about, writing about, or looking up ideas for teaching, I'm hanging out with my hubby, son and minpin Lily.