I'm not very familiar with the K-2 units,but here's what I think about 3-5. In terms of 3-5 I think Landmarks in the Hundreds (3), Landmarks in the Thousands (4), and Mathematical Thinking at Grade 5 (NOT the same type of unit as the Math. Thinking unints in grs. 3 & 4) would be one good thread. These units cover lots of ground in terms of unders system, whole number operations, and multi-step problem solving. The fractions units, Fair Shares (3), Different Shapes Equal Pieces (4), and Name That Portion (5) would be another good thread. These units really build upon each other to give kids many different models for thinking about fractions, decimals, and percents. There are certainly other threads that could be used, but these hit on some really important math topics and they provide a wide range of experiences for the kids.
