I have the small ring agenda & love it. I keep credit cards & other cards in there. Also receipts. I write down all my bills in the calendar section - that way I see when things are due & am never late. I keep track of when & how much was paid. Also the normal appointments, parties, events, days off, OT worked, birthdays, vacations etc.
I use the note pages to keep all my online ids & passwords since it's become impossible to remember them all. They're kind of in code in case it was ever lost or stolen.