There is always odd ball yarn you can make with the extra. Just tie them end to end and then knit something. Make sure the yarns you use have the same washing instructions. This is ideal yarn for baby blankets, afghans, dishcloths (if cotton yarn), etc. Anything, really. I don't even think it needs to be the same size, well, maybe similar (fingering, worsted, bulky, etc). You don't even need to weave in all the ends - just cut the ends at the knots to 1 inch and leave them hang out.
Just how do you plan to use the yarn for cards?