During the worst depressions, I have always found that it's helpful to recognize that there are people who are--or have been--in worse shape than I. Helen Keller frequently comes to mind, and I love this quote from her book. "I long to accomplish a great and noble task; but it is my chief duty to accomplish small tasks as if they were great and noble."
When I can barely function, my memory is shot, I can't read, I don't feel like going online, I can't concentrate on anything I care about, and feel totally lousy, I have always tried to find one small task each day that I can accomplish.
Sometimes, it is a baby step like sharpening the pencils on my desk, going through my change and putting 50 pennies into a roll, writing one email to a friend, walking outside and sitting in the sun--even if it is just for 10 minutes--running a load of dishes through the dishwasher, or reading a children's book.
One activity I have always enjoyed is "handwriting." Years ago, I bought a writing tablet, which I keep by my bed. On my worst days, I take it out and just write my letters. I have lovely penmanship. Cursive writing reminds me of simpler times and childhood memories. The physical act of writing makes me feel good.
Although these activities seem negligible when I'm well, I've learned that the smallest step forward is important when I'm ill. As Carl Rogers said, "What I am is good enough if only I could be it openly."