Hang strings of beads along the edge of the area underneath the stairs for a funky 1970s look. You'll disguise the empty space from view and create a private alcove underneath. Hang a reading lamp on the wall underneath the stairs and stash a beanbag chair in the space behind the beads to create a secret area where your kids can sneak away to curl up with a good book.
Build bookshelves under the stairs where you can store your favorite novels, reference books or CDs and DVDs. Make the outside edge of the bookshelf flush with the edge of the stairs and completely cover the empty space. Paint the shelves the same color as your walls so they'll blend seamlessly with your existing decor.
Create an indoor garden along the edge of the empty space with tall plants and indoor trees. Select pots filled with ficus trees, palm trees, large cacti or leafy dieffenbachia and arrange them in an attractive way. The foliage from the plants will draw the attention of anyone who passes and divert their eye from the space under the stairs.
Build a wine rack underneath the stairs where you can store your favorite bottles of vino. This works especially well for stairs that lead into a basement where the temperature is cooler and is more conducive to preserving wines in an area where they won't become overheated.
Install a painted wooden lattice along the edge of outdoor stairs that lead up to a deck. Leave the lattice as is, or plant ivy at the bottom of the lattice and train it to grow up the wood as a natural cover for the space under the stairs.