First of all, I like the 5 x 8 ruled index cards for planning, though I do use the smaller ones (3 x 5) for other things. Where you would just put down one item on a smaller size flashcard, you can put a list or a series of things on a larger one.
I started using these bigger index cards, which I found at the college store, in the San Francisco hospital when Aidan was in the hospital waiting for his liver transplant. They were easy to carry and handle and write on with one hand while I was rocking him in the NICU rocking chair.
I wrote chore charts, and copywork from my Magnificat (prayers and scripture) and lists of things to remember. Later I could color-code them or add supplementary notes on the back.
That is pretty much what I am doing now. Basically, anything I want to remember, or look over in future, goes on index cards.
Some virtues of index cards are that they are sturdy, easy to shuffle and re-arrange, and portable.
Another good thing about them is that they are half-size to normal paper, so I can easily scan them two to a page and archive them/print them out in paper form.
Here are some examples of how I use them, in no particular order.
For Overview or Big Picture Planning.
Purple is for "overview". Here I wrote out my ideas for handwriting for this year for the three boys. My highschooler doesn't enjoy writing very much, but I thought maybe I could get him interested in calligraphy, especially as it has implications for graphic design, according to Steve Jobs : ) Plus calligraphy is an old fascination of mine, so I have lots of books and resources collected.
Here is another overview. Here I wrote down course notes for Faith Studies for Paddy, who is in 4th grade. I decided to have him go through Living My Religion 4. I haven't used these books before, but I like them. I studied the Teacher's Manual for the courses and wrote down a few details on the goal for the course, possible supplementary resources, and activities that could go along with the readings.
For Specific Planning
Yellow is for proximate plans -- things I am going to do in the next week or two weeks. These two cards have to do with Aidan. Aidan is 13 but functions academically somewhere between K and 2nd grade. I wrote a little checklist (above) to remind me what to do with him -- his work is mostly not book-driven but hands-on, and I always forget to work with him if I don't have some reminder.
Here is another yellow one, I have been writing down daily plans for our Morning Time. I found that if I didn't write it down in detail, I would forget and end up missing something or else flipping through my books while my boys waited ;-). If I can learn to wing it better I will stop writing out the plans, but since this is only Week 3 of our summer term and I am a slow learner, I am still trying to get used to the routine.
For Temporal and Liturgical
This is an example of using an index card for copywork. I wrote out what Saint Ephrem the Syrian had to say on Prayer, from my Magnificat. You can find the quote here if you are interested. This had two purposes. One, I wanted to remind myself to pray always and everywhere. Two, "how and when to pray" is one of our summer threads or themes, something I want to talk to the boys about, so I thought I could read it to them sometime during Morning Time.
Green is for Temporal -- sort of an odd, intuitive category. I highlight our schedules in green but I also use it for spiritual/liturgical resources.
This is a scattershot post, but I hope it gives you a few ideas on how you can use index cards. Some other time I will post more (these were the photos that turned out best -- the light in our house can be challenging sometimes especially with white things)