Use the number line for rounding. Generally (but not always!), we teach children that 5, 6, 7, 8, 9 numbers are rounded up to the 10, with 4, 3, 2, 1, rounded down. The number line is essential in teaching this concept. Is 4 closer to 0 or to 10? Is 37 closer to 30 or to 40.
Don't forget to use a number line vertically, as well as horizontally. We read number lines for graphing, and for grids. A protractor is a curved number line, and children have to read these with zero on the left or the right, for angles whose vertex is on either side.