A calendar embedded in OS X desktop that allows me to cross off days I workout - does something like this exist?
Use the build in calendar (came with osx), open it, click on the (+) sign to add new event for today, type worked, then you have lots of options to use if you want to, but that is the simplest way to do it.
You can even make a new calendar specific to your activity.
You can email the event to yourself, etc...