A great introduction to systems of measurement is to have students create their own. Ask students to create referents for measuring small things (like dimes), medium things (like the width of their desk), and large things (like the distance to the next town over). Students will likely use body parts as these referents, which is great, because they carry those referents with them wherever they go. It might be handy for them to know that the width of their finger is about 1 cm, the length of their leg is about 1 m, and so on.
You can give the students a sheet to track their invented system and compare it to SI and Imperial, which is likely where you are headed next in this unit. This is an example of such a sheet, in Word format. Referent Chart