This is the pocket chart I use. I know the glare is bad in the pictures but bare with me. I use colored index cards to make my numbers. The green are for changing the number to a rounded number and the yellow are what I use to make up the numbers. Across the top I have the place value cards. Right now we are just doing hundreds, tens, and ones. The white cards towards the bottom are greater than, less than cards. I sometimes throw that in there for practice.
Here I have set up two numbers using my yellow cards. We are practicing rounding to the tens place.
Here you can see the rounded numbers. The child comes up and places the green cards over the yellow cards that should change when rounded.
This is such a simple activity but it has proven effective. I used this same method when I was teaching 5th & 6th grade math at the Christian school.