One thing you can use card stock for is scrapbook leaves. The standard blank white pages that come with most scrapbooks can seem dull, especially if you are an artist and like to use a scrapbook as a place to keep photos of your work. Instead, paint, draw or print imagery unto the card stock and then use a three hole punch on the card and place it in your scrapbook, ready to use.
If you want to create something you can share with your friends, paint or draw an image and scan it into a computer. Or use a graphics painting program like Paintshop Photo Pro, Photoshop or GIMP to create the image directly on the computer. Then print it out tiled across a sheet of card stock paper. Cut the cards apart and mail them off.
Another fun project you can do with card stock paper is to create animation flip books. Cut the card stock paper into squares about 4 inches by 4 inches to produce 20 individual cards. Then draw a scene on the first one. Draw the same scene on the next one, except that you change something, such as the position of a person. Continue this to create a series of cards showing a person or object moving in the sequence. Then use a hole punch to punch holes in the tops of the cards. Then wire them together in sequence to produce your flip book.
Paint or draw figures and objects you want to use as parts of a mobile onto sheet of card stock. Use scissors to cut the figures out. Punch a hole at the top of each figure and put an ornament hook through it. Assemble a wire framework and hang the figure from it. Then, hang your mobile from the ceiling.