This is a link to a page that contains Challenge problems and Advanced Challenge problems. These appear to be problems appropriate for Advanced 5th grade (maybe even 4th grade) and up.
The page includes a 30 point rubric for each problem, with only 6 points of the rubric going to having the correct answer. It focuses more on showing your reasoning and writing a concluding paragraph.
Take this link to the home page and then click on either 'Challenge problems' or 'Advanced Challenge problems'