Any ideas how to hide this vent?

Hi~! A wood decor panel (like the link I've attached) would work well. You can buy these for a reasonable price online or Home Goods has some at reasonable prices! (check out the clearance section!!)

https://www.homedepot.com/p/Benjara-Brown-Rectangular-Mango-Wood-Wall-Panel-with-Cutout-Scrollwork-Details-BM01886/312154251

Here's some decorative ideas for you!

https://www.shelterness.com/diy-return-air-vent-covers

Paint with wall paint or hang a decorative mat that is open like this one:

https://www.walmart.com/ip/DII-Rubber-Scroll-Doormat/181827348?wmlspartner=wlpa&selectedSellerId=0&adid=22222222227286563366&wl0=&wl1=g&wl2=c&wl3=345322886064&wl4=pla-711241697680&wl5=9007582&wl6=&wl7=&wl8=&wl9=pla&wl10=8175035&wl11=online&wl12=181827348&veh=sem&gclid=EAIaIQobChMIw9DAiqX36AIVTl8NCh07fwKTEAYYCCABEgLwdPD_BwE

Painting it the wall color would help a lot.

You could try using a decorative vent cover like this: https://ventcoversunlimited.com/decorative-resin-wall-or-ceiling-vent-covers/

I would put a picture or other object to cover the vent, but have it set off the wall to allow air to flow around it an into the vent.

A cheap and easy fix would be to paint the vent cover the same color as the walls. That would allow it to blend into the wall more and stand out less. I did this on a cord that ran down the wall from a clock. Once the cord was painted it blends in very well.

How about painting same color as wall Then put a shelf with some collectables that will camoflage without blocking the vent. Maybe put decorative metal on the screen too

Painting it the same as the wall color would be the best solution. Covering it with anything decorative such as suggested will reduce the air flow by 30%. The vent already has vanes on it and sized for air flow to the equipment. You can try but if you hear/notice in the furnace/blower speed up remove it. The blower is straining and can fail. Also check if the air flow in room supply vents is reduced.

Paint it the same color as the wall so it disappears. While it may be unattractive to you, a crucial component for any HVAC system. I would rather have a vent over no HVAC at all. A necessary evil in a modern home.

You could incorporate the vent into a faux painting of a window. It already looks like a shutter. But because the space is so narrow the vent would be a closed shutter and the other painted/faux shutter would be mostly opened to the outside. Google "trompe l'oeil window" or search in Pinterest.

GoI the "easy" route (to avoid hand painting it all) and get a stick-on mural of a beautiful view or get a picture enlarged. Attach it to the wall so the vent (closed shutter) would cover half of it and cut away that area. Take a picture of the vent at a very steep angle for the right perspective, get it printed, cut it out and attach it on top of the "view" in a way that this shutter visually opens to the outside. Then put a window frame around all of it. You can even attach a narrow window sill and put a flower pot on there for more realism.

To make sure you would like the end result before committing to it with professional printing costs you could print out a view and the angled shutter on paper, tape it to the wall, then stand back and look at it.

Check out the picture #6 at this website that kind of shows what I'm talking about. But instead of looking into a room your window would be looking out. I would make the opened side farther open, so you have more of the view visible.

http://www.sashrepairs.co.uk/info/false-windows-fancy.html

Instead of covering this important piece you would make it part of an actual design feature in the room.

Kirklands has some very nice metal decore, [with holes] that you can use to cover it and let the air flow.

Also Hobby Lobby, and the hardware stores sell decorative aluminum you can use to cover.

This will sound crazy. Can you get another matching vent to place beside the existing vent. The two of them together would then be centered on the wall. The existing one calls attention to itself by being off center. Then frame both with a wide decorative frame. This will give you four panels that you can paint. Paint wont hurt the air flow. Not sure how artistic you are. You could do something simple like four trees or four flowers. Or something intricate like a Baltimore screen painting.

https://www.google.com/search?q=screen+painting+baltimore&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiNmsCDtvjoAhVWAZ0JHStQB0kQ_AUoAXoECBMQAw&biw=1093&bih=498&dpr=1.25

I forgot about the decorative metal panels. You could frame a large metal panel and do your painting on that. Just make sure not to block the holes.

Hang something with holes over it like an old wooden shutter

Put a large picture over it or a mirror

How about some strands of beads or a macrame wall hanging?

I would paint it with the same color of the wall, it will not disappear but it will definitely look way better...

Spray paint it gray to match wall. Once you put latex on those grills, they get hard to dust.

Then a bigger decorative screen over the whole thing, centered.