I would not put anything opposite the collage you have now because your hallway is too narrow. Some people like to move as far away as possible from such displays to take the pictures in as a group. That wouldn't be possible in your space if you had stuff on the opposite wall--- they might back up to see one wall and knock things off the other one. In general, if you have a hallway width less than four feet wide you don't want to line both walls with anything. If you have a long hallway or one punctuated by doors, you can put a grouping on one side, then hang things on the other side of the hall, not opposite but further down, iykwim. I would use a nice painting or print at the end of the hall, unless there is something you want to reflect from the other end.

Rental center is a really good idea- they not only have columns, fountains and stuff, but they do this all the time, so they probably have a ton of ideas and you don't have to re-invent the wheel. If you don't live near a rental center- go to a website and get ideas. Your florist may have some sources for decoration ideas as well. I don't know if you like balloons, but a balloon arch is a very quick way to make a large, colorful statement. You can also float them as a kind of false ceiling, with silver or gold mylar ribbons on them that catch the light and sparkle. Nothing says "party" like balloons. I recently went to an outdoor wedding where they wanted to keep insects off the wedding cake, so they rigged a tulle "tent" over it- gathered tulle onto a tree banch and let it fall gently all around the cake table. It looked really nice (and also gave the message to stay away from the cake until it was time to cut it.) Good luck and enjoy your wedding day!

My hallway has our bedroom door at one end, the bathroom door on one side, and the opening to the living room on the other side (staggered, not directly across from the bathroom door), so my wall is only visible from the hallway. It's only a couple of inches from the bathroom door to the end, so I don't have room for furniture. I built narrow shelves the width of the hall, for family photos. It's easy to change out and rearrange pictures, and they are always straight! Ugh, I was pretty lopsided when I took this.

If you have a pair or trio of small pictures that work well with the mirror frame and table (or a small wall shelf or box (perhaps with a door or drawer for hiding a cell phone and/or keys and/or hair brush for below the one small picture), you could leave the mirror vertical and hang it eye level but off center horizontally with a grouping of smaller items beside it. You might get some poster board and cut it to the shapes of your mirror, etc., and then use removable tape to test how they'll work spaced in different ways.