Remove any vents with an unnecessarily large border and replace them with smaller, more discrete vents. Fill in and paint any screw holes that are left exposed after replacement.
Paint your vents the same color as the wall. This helps them to blend in better if you can't hide the vent completely.
Place seating around a vent that is located on the lower portion of the wall. Don't put a big couch right in front of the vent, or it will stop it from being useful. However, a solution such as a bench with open legs could be placed in front of the vent, effectively hiding it while allowing air to still flow into the room.
Put tall bookcases or other tall furniture underneath higher air vents so that they almost come to the height of the vent. Then, you can put knickknacks or picture frames on top of the furniture to further conceal the vent without blocking it. Another option is to cover the vent with an open-backed book case. This will take most of the eye's focus away from the vent without blocking its air flow.