Credit cards, ID, drivers license, keycard - whatever - under control. This thing will hold 1 card as well as it holds 5. You can stuff in some cash too. You can probably stuff in a key. Or a guitar pick, or a toothpick, or a q-tip or a golf pencil... you get the idea.
Step 1: The stuff you need
2 inch tubes work the best, like from a mountain bike or a kid bike. Mine is from a fancy stroller - so it is probably better than yours. Any bike shop will almost certainly give you one if you don't have one.
2. thin card.
Use any thin credit card-sized card. Paper ones won't work, but anything laminated or plastic should be fine. AAA cards, library cards, gift cards, etc. Find one as thin as possible to avoid extra bulk. Mine is a NYC metro card so that everyone knows that i used to live in the city and I am still that cool. I think it even still has money on it - I don't even care.
3. double sided tape
The stickier the better.
Step 2: Cut the tube
Step 3: Cut and stick the tape
Step 4: Remove the tape backing
Step 5: Stick it!
Step 6: Done
Yes, that's right it will eventually rip and you'll have to make another. Mine typically last about 6 months with 4 cards. But that is less a testamant of the product and more of my own awesomeness.
PS - I've been using it for a year and no problems with credit card magnet stuff.