Plain & boring reception hall, help !

The initial idea I have is that you get either wrapping paper or wallpaper and use double stick tape (or poster mounts) and maybe put these up maybe in strips separated by 6 foot distance. You could possibly also get a wedding themed picture/poster or several. It can help reduce cost by just hanging these with emphasis around the wedding couple's part of the room where most eyes and hence most pictures will be directed toward. Part of this concept is to direct people's attention to a wonderful looking area so that the plain areas are less of an issue.

Another option could be to get some live plants to place against the wall. I do see that you may only have weeks to figure this out. Hoping the best.

I think the walls are better plain white versus a hall with old, grungy wallpaper or dull paint. When the background is white, a person's eye then focuses quickly to center stage where there is color and/or movement. So place your sweetheart table, king's table or band/ DJ booth at the far center of the long room and place a colored backdrop behind. Some ideas are wall or standing candles, chandelier, plants, fabric backdrop, custom gobos projection light. You can keep the remaining centerpieces and decor as is. One simple solution can have the most impact without calling attention to what you Don't like about the venue. Guests will ultimately focus on you (the beautiful wedding couple), other guests, the entertainment, and the food in front of them-- everything that is lively and colorful.

To offer an idea, I would reccomend maybe the light projectors that could be places along the floor and project something onto the walls, or there are frames with floral or greenery as a back drop that can be found pretty cheaply on Amazon.

You could just do colored lights on projected on the walls to match the wedding colors as well. The light will add definition and texture while helping with the lighting... I know your reception starts at 3pm but if it's indoors you could lower the indoor lighting to allow for projecting lights to appear better on the walls.

Definitely play with lighting, amazon sells cute Fairy light curtains You can also get some large marquee letters with you and your partners initials. Amazon also sell some fairy cork bottle light you can get different colors that would like nice on tables or on a bar. Bouquets of baby breath and candles/flamesless candles can also make cute table decor. The most important thing to remember is nobody is there to see the venue but rather to celebrate you. Fill that room with love laughter and fun and it will be an unforgettable night. Congrats!