When I was about to throw away my cards (lost a few cards, and it needed replacement anyway), I thought of making this: a pack of cards with a hole in it to hide things in. It can be anything (as long as it's small). How about some spare money? Maybe a few batteries, or a mini knife. I use it to keep some spare money with me.
I used pieces of leather from an old wallet to line the inside with. It looks very neat, if I say so myself.

Note: don't underestimate the amount of time this will take. Took me about 2 hours. You have to cut and glue every single piece separately.

Second note: remember to be careful with knives!

Step 1: Materials You Need.

-A pack of cards, obviously
-A ruler
-A sharp knife. I recomend an xacto knife, but I used a small spyderco knife because that one is actually sharper than a xacto :)
-Scissors
-A few pieces of leather, or felt. You can probably use a lot of different materials for this. Or you can skip lining it at all, since that saves some work.

-A cutting board might be a good idea, since you will be cutting with a sharp knife. Don't want scratches on your table, even if it is a workbench!

What I forgot to include in this photo is glue. Very important :D
Any kind of glue is okay, I guess.

Step 2: Cutting the Cards

You want to keep 17 cards for later. Just lay them somewhere else and forget about them (for now, just try to remember where you put them though).

Proceed to cut out the centers of all the other cards. You can see how I did that in the pictures. I didn't have to draw or anything, since the picture on the back is a great guideline on where to cut. Instructables can't show gif pictures so you can take a look at this:
i156.photobucket.com/albums/t26/eygen/6.gif

If your cards don't have this pattern (which I kind of expect), you have to draw the cut out pattern, or it will look very sloppy if you do it by hand.

Repeat until you don't have any cards left (except for the 17 cards you just put aside).

Step 3: Glue the First Part Together

Should be obvious. Glue the cutouts together. Don't use too much glue, or it might get soggy.

Enjoy lunch while it dries :)

Step 4: The Bottom

To make the bottom you can glue two (of the 17 you have left) cards together. On that, glue a piece of leather, or any material you have decided to use. If you don't want to do that, that's okay.
For anyone that does, the leather piece will need to be bigger than the parts you cut out. Because of that, there will be a space left by the leather. You need to cut 4 cards like in the picture, and glue them around the leather. You now have 11 cards left.

Step 5: Putting the Leather on the Middle Part.

You need 4 pieces of leather (or like I said, any other material), to line up the inside. You can see how I did that in the picture. I used glue for this too, and it works great.

Step 6: Time to Fill the Gaps

Because you've added another layer, just like with the bottom part, you need to add cut outs to fill the gap left by the leather.

When you've cut out 8 of them, you can start assembling. You now have 3 cards left.

Step 7: Finishing Up.

You have 3 cards left. Like you saw in the picture, cut out one of them, and glue it on top of the entire thing. You're done!

Now you have two cards left to cover it up with. I haven't figured out a good way to cover it yet. Suggestions are welcome! I was thinking you could use a pin to make a small pivot for the two cards on top, but that will come apart too easy.

As you can see, it's almost identical to a normal pack of cards. It still fits in the same pack and it weighs less, even with a battery!
I think this is a good way to always have spare money with you. Use bills though, no coins, since they make noise.

Step 8: Suggestions

Edit: sweet! I got featured!
Edit: Sweet! It's in the popular list!

Any suggestions, comments or questions? Don't be afraid to comment. Go on. You know you want to.

Don't forget to rate too :)