Family pictures or if you don't want to go that route; a big awesome painting

What about paintings then? Or a mix of pictures and things your into? Or you could make it a statement wall with wallpaper

The second picture is cute (with the 'hi'), I would put something like that set up behind the door as it opens but keep it simple as that area will get crowded quickly; if you're going to hang a mirror place it on the side the door opens on so you and guests can take a quick peek at yourselves as you head out.

If you're going to decorate the wall that looks like it's heading downstairs space it low enough that the two walls aren't competing with each other. I would do one wall busier and the other more simple.

A little outside the box but kinda cool and there are lots of variety of shapes and colors. They just screw into the wall in your own formation but be warned, they're not exactly inexpensive.

There isn't much room to add furnishings; however, if you were willing to crowd the space, you might add a coat/hat hook on the left side of the door hinge.

Better yet, just add wainscoting to the walls, perhaps with art above on at least the wall on the left as you walk through the door, so they so they will not look bare and add your storage a few steps from the door ... beneath your stairs.

http://www.designsponge.com/2016/05/12-storage-ideas-for-under-stairs.html

If you are unable to create under stair storage, on option might be modern corner shelving:

http://decoholic.org/2012/11/27/14-best-corner-shelf-designs/

You could have as many or as few shelves as suit your purposes. My thinking would be to have them begin at the baseboard and have the top shelf no taller than the door knob. You could paint them the same color as the walls to help them visually disappear.

Your inspiration pics with plants have doors that allow light in, which a real plant will need to survive. If you want a plant, it will need to be artificial, unless you are changing your door to one with windows.