Draw giant system with chalk on the ground and have students move through it
Dissecting one is always a great time
CEV has some decent videos of explaining the system
I have students make a digestive system out of random materials from home and they have to label it with the functions. I think they enjoy it. I have had some pretty impressive projects over the years.
Create models from play doh/clay
models from random objects that they bring in
dissect one (get it from a butcher shop/processing plant/a student that hunts)
if you can get access to one- introduce the students to a cannulated animal