If they understand the concept, which I assume they do, since you've already used practice sheets. Make two spots on a table, one is for countable items and the other is for uncountable. Give the students several small slips of paper each and have them draw pictures of items that are uncountable. When a student finishes a drawing have them hand it in then poll the class as to wether it is really countable or uncountable. Then place it in the proper spot on the table. Always accept any drawing, but if it's wrong we want to make sure kids understand.
You can start with your own drawing. I like to draw poop. Yes, poop. As an adult poop discussions are rather taboo, but young kids freakin love it. They die laughing and become really interested when they see my stinky soft cream picture (sorry so vivid.) I liven it up and try to give it to some of the students or ask them to smell it. Interestingly enough, nobody wants it nor do they want to smell it.
Then you can show them a drawing of a red puddle. Ask them what it is? "Do you think it's snake blood? Is snake blood countable or uncountable? Actually, it's wine. Is wine countable or uncountable?"
Hopefully, you'll get pictures of salt, love, sugar, (I always get pee,) etc. If you've done practice sheets, you'll get stuff from those.
Anyway, I think it's a fun way to have them produce and brainstorm ideas for uncountable items.
I hope that helps.
Mark
www.mes-english.com