I've been thinking about doing this recently too. I was wondering about just printing, cutting out and laminating some kind of little shape or thing Ollie is into at the moment (mostly vehicles!) and then sticking velcro dots on the back so that they are reusable. If I do trucks and things then I might just make a road or carpark or something for them to be stuck on to as the 'chart'. What is Ella keen on? You could create a flower with a long stem that she could attach leaves or petals to, or a ladybird you could put spots on, or a fairy wand that could have little stars around it...? I think just about anything could be adapted.
The other thing I had thought about was making my chart a bit more specific (haven't got to the detail point yet) and creating a wee list of chores that he can start doing (with help). Things like putting away toys, putting clothes in the laundry basket etc. Then he can see quite clearly what he is getting the star for.
Oh and I might be a mean mummy but I hadn't even thought about there being a treat at the end. I figure at two the treat is getting the star (or whatever) in the first place!