How do you attractively cover a 2' by 3' metal air vent?

I would put some type of picture or art work over the vent. However, you will need to allow air flow, so put a spacer between the art and the vent.

You can build a frame out of wood and paint or stain it then use a piece of lattice or wire mesh (see pic).

Yes those vents are not always placed to be aesthetically pleasing to the eye. I have always just painted them the same as the wall to make it disappear as much as possible. Another option is to use a decorative mesh screening and then frame it to make it look intentional. Before going this route, check with your HVAC service to make sure there will be proper air flow.

Have an HVAC guy come out and give you a quote to relocate it.

You could also hang a fabric type hanging 6" out from the wall from the ceiling on a bamboo pole.

Do ask your HVAC person....we had a new air conditioner/furnace installed and they advised us to take one door off (in the basement) and put two vents in another door and two vents in another wall (lower) for air flow. I think painting your vent the wall color would be the best. You could maybe hang a skinny metal sculpture over it and then put two sconces on either side to make it look intentional. Maybe your HVAC person can give you some pointers about what others have done.

You could hang a curtain over it and pretend it’s a window make it look like a window

You can get a sheet of perforated metal that's typically used for radiator covers, trim that up carefully, attach it to the air vent. You might consider a frame or edge to keep folks from getting stabbed by the cut edges. I find it's best to highlight, if you will, an eyesore as it will then recede and no longer be annoying. You could also paint the vent to match your wall.

how can I do this?

how can I do this?