Thanks Dave and John for your comments. It's definitely not set in stone yet, and suggested improvements are most welcome!
Regarding whether to have past-tense versions, if 'To do' and 'Done' are styled sufficiently differently (as in the mock-up), then I don't think a past-tense version is necessary. If I have a to-do list, when I have done something, I just write 'Done' next to it; I don't then go and write it out again in the past tense.
I also wondered about reversing things as you suggested John, however the colon in things like "Receive a grade: Done" or "Make entries: 2: Done" doesn't seem quite right to me.
For simplicity, I like having only the styling and the words 'To do' and 'Done' changing.