Can I tell you how much I love Singapore strategies for math? Their place value cards, which I wrote about here, are absolutely amazing and have solved so many regrouping, expanded form and number sense problems. Another strategy I've fallen in love with are Part Whole cards, which look like this:
The top part is the whole, and the bottom sections are the two parts the number can be broken up into. Each card has a missing section and the kids have to use mental math and addition or subtraction to figure out the missing number.
In 2nd grade, I used these to build fluency with math facts, and to help kids understand how to compose and decompose numbers. They also came in handy while we were walking to the lunch room or waiting for an assembly to start (the kids would show me the missing number on their hands). I even had struggling kids make their own out of index cards and take them home to practice.
In fact, I used them so much and they were so helpful, I decided to make some I could use now in 4th grade. So I created ones that used multiplication and division to build those facts. I'm excited to try them out next week with my kiddos!