Oh goodness I remember those days LOL! I used to ride my sisters bike because it didn't have that "ahem
-Buster-bar" down the center, besides, she had outgrown it. It had a bananna-seat, high handle bars and that extended, inverted U seat bar up the back
I rode that bike into the ground.
Andy said the same thing I was going to say, we used baseball cards, but then that was back when you could get a pack of 10 (with a piece of gum you could shave with) for about 25 cents. Now-a-days they're "Collectables".
Thinking back it seems I used a clothespin too, clipped to the bar that held the mud guard on but then the forks kids have now-a-days are a lot thicker and most don't have front mud guards anymore.
You can pick up a automotive hose clamp at any Wal-Mart (and even in most grocery stores) and once you screw it down, leave the "tail" piece sticking out where you can clip a card onto it. It might work. Either that or get a longer clamp and let the tailpiece stick into the spokes.
What kind of bicycle is it? If I could find a picture of it on the web it might help us figure out an option for you.
~ Raven ~