Healthy/Clean eating challenge?

Does anyone know of a good 28-day challenge I can do to help motivate myself with eating better? I already cut out stuff like sugar/most sweeteners, coffee, dairy, some meats, (I do have a problem with original potato chips, honey, pb and coconut oil) but I would like to fine-tune my diet. I do better with challenges that have a start/end date. I have a few races coming up, so I don't want extremely cut out carbs or anything like that.

Suggestions? Anything you have seen or tried before? I'd appreciate it!