How do you fill empty hallways and walls?

Like others have said, narrow tables with decorative clutter are always nice to add a little extra to an otherwise bare hallway. A mix of decorative wall-hangings and wall mount shelves with decorative clutter works well for narrower hallways where even a narrow table just won't fit well, or combined with the table(s) in hallways with more room. Decorative plants strategically placed and the aquariums, like @Karababy52 brought up...they actually work very nicely in hallways. I also like the bench-seating for hallways, i believe the ones I use most often are from Spa Day pack.
Mirrors also, are very effective, especially if strategically placed. Whether in a hallway or a room, mirrors can help to make a space seem more open, they are more than just a decorative item.

Generally, I tend to opt for wider hallways that serve as both hallways and have a combination of things that make them more than just a hallway. I will make rooms smaller just to make sure hallways aren't very narrow if necessary because hallways can be so much more than just a hallway if you give them enough space.
I have two very long hallways in the penthouse my active sims currently live in and use a combination of all of the things mentioned in this thread for them. One of the hallways in my current home is wide enough that I have a bar and counters running along the length of it, so despite it being a "Hallway" it is also a room for entertaining guests. The other hallway is on the second floor, and it has a wider area around the stairs up that hosts the computer desk and a game table, then narrows into a more proper hallway with just a runner, shelves, decor, etc.

Sometimes less is more! But, I use mostly artwork and I do like the idea of windows. Also sometimes decorative wallpaper helps.

@Stormkeep I love the way you decorated your penthouse! The bar idea is really inspiring and since one of my young adults is moving into a penthouse at some point, I'll definitely keep it in mind!

Wow! @Stormkeep That is an awesome place :smiley: I really like what you've done with the area with the bar.

Well, my go-tos are paintings/posters/tapestries for walls and a nice sidetable/commode with nicknacks and/or plants/statue. Also elongated (is that the word) rugs that run through the hallway like Stormkeep has done. What are those rugs called anyway? I also, like Stormkeep, make use of the space if the hallway is big enough to fit the more functioning items to make it useful. For instance I really like the little workspace (the desk/computer/chair) on the upstair landing at the Partihaus' place even though every bedroom has a desk and a computer.

In one place of mine a hallway is sporting a threadmill and another holds the family's geek doll collections and if there is a frontdoor or even a backdoor, I do like to put a working or even non-working wardrobe near them for all those outdoor clothes, especially since I got Seasons. A doormat and the umbrella stands are also a must.

The hallways also come useful when the household expands and space is premium. In one house the upstair hallway is where I put stuff that are stored. You know, every house has a place where things pile up but are too useful to throw away. I put down a nice empty bookcase and filled it with all kinds of cc-clutter like a flashlight, boxes, some bric-a-bracs, old dolls and toycars, a few of those magazine holding things, gloves, a wrench and other mechanic things, gardening tools and even books.

This bookcase was actually in a room that functions as a man-cave/storing room/an excersize room and it even had a woman's vanity table because the bedroom is too small for one. But when the kid came long the bookcase went into the hallway so I could fit a toychest and a few kid related stuff in preperation of moving the kid in when he becomes a child. Right now he sleeps in the same room as the parents.

Hah, thanks @Stina1701A, the original builder gets alot of credit for the decor, because they already had a bar in the hall, I just replaced it with a different one, added more counters and some decor.

The long rugs are called "runners", by the way, at least where I am from.

There are a few "family photo" style art works in the pictures category. I often use them in hallways because its what my mother did when we were children. Our family photos lined the hallway that connected the living room to the kitchen and dining rooms. It just feels homey to me.

If the hallway is wide enough, I'll add a few armchairs or a coffee table with lamps or knick knacks on it or the large floor setting vases to decorate them. Hallways aren't very functional besides just connecting rooms, so I just try to dress them up a little and make them less clinical looking.