How about some vintage items, like small trunks or large tin containers with some vibrant colors? Also I would add more to the wall of pictures, a large wood or metal letter of your last name and maybe a big homemade chalkboard with your favorite quote on it.

I would remove those plants and use some vases and candles( art pieces would look good also)

To accent that area I would first move your family photos to another area and put a larger picture or photo there. Then pring in some ceramic vases or sculpture on the open areas of the staircase.

Lucky you to have such a great stairway to work with! Take advantage of this. It's not just the plant shelves that you need to work with, the whole area could be so much more. Those tiny framed photos aren't doing anything for the space. You need to redo those collages to follow the geometry, and go much larger with matting and framing. Once you get that right those ledges won't matter as much.

great advice from patriciarhall. One large art piece would give a solid base to your heavier shelving above and would be less busy. Grouping all the photos on the landing wall makes good design sense - try to have the top row of the tops of your photos placed in a straight line so your eye travels straight across, staggering the rest of your collage of photos as mentioned above. Play with it on the floor before hanging and hang at eye level height. (Center of collage at about 5 feet high.) Tall glass vases, pottery, etc., to coordinate with colors of artwork will balance out the weight of the artwork and create
a natural flow up the stairway. Have fun making your stairway a wonderful focal point to your home!