I wonder if something like Life of Fred would be workable in your situation, here -- since it's written like a story, but with math problems at the end of every chapter, and it doesn't at ALL look like worksheets so it won't appear to be mimicking "real math lessons".
fwiw I started a challenge math program for the whole school at our elementary. They get a new sheet every other week and kids get the chance to have their name drawn for a prize if they finish the sheet, so if it was our school, I'd get the teacher's permission to go over the week's math challenge sheet with the kids who finish.
