The simplest solution is covering the vent with a painting, poster or picture. You can also try framed objects (leaves, coins, shells) and more unconventional wall art such as found items (driftwood, tiles, pieces of metal). Hanging fabric on the wall over the vent is an option, but remember that you don't want to keep air from circulating. Thinner fabrics can be nailed up, but for heavier textiles such as rugs or draperies, use rods and hanging systems that keep the material away from the wall. Another creative solution is placing a faux window over the vent, which also creates a focal point for the room.
Strategically place large furniture to cover an air vent. In the bedroom, an armoire, bureau or wardrobe would work nicely for this purpose. In the dining room, a china cabinet or buffet. In the living room, a bookcase, grandfather clock or tall, potted plant or small tree. Folding screens are another good option; they come in all colors and styles from elegant Japanese shoji screens to inexpensive ones you can make yourself. For vents that are very high off the ground, rest a floor-to-ceiling mirror against that wall, which also has the added benefits of reflecting light and making the room seem bigger.
Try wall treatments that trick the eye by making the vent less noticeable. Paint the wall one color, then go back over this base coat with random blocks of color, "hiding" the vent in one of the color blocks. Or do a faux finish on the wall, applying the finish to the vent as well. Or put a mural on the wall, painting right over the vent. It will almost disappear in the beauty of an underwater seascape, or city skyline, or desert oasis or whatever scene you choose. You can also purchase ornamental versions that look more like decoration than air vent covers.