Make a list (your child can help) of items that your child can measure. For the holidays, for example, the list can include some of the following: light bulb, candy cane, ornaments, wreath, popped popcorn and cookies.
On a sheet of paper, help your child make three columns: Object, Estimation and Actual.
Have him then write the names of the object he's measuring in the first column.
Prompt him to guess how long or the item is. You can use inches or centimeters depending on what he's most comfortable with.
Once he's done estimating each item, he can actually measure each item and see how close he was.