Depending on your style my first suggestion would be a large framed picture. If you have a hobby lobby in your area they are great and have many options! They always have sales and if not you can use the weekly 40% off coupon on any full priced item by downloading it on their app! If you don't like a picture, maybe do a framed photo wall collage. I'll put some examples.

for more pictures search hobby lobby wall decor

for more ideas use pinterest and search framed picture wall collage

Your question said 'up the stairs', but do you really mean to place the pictures on the wall of the stair landing?????

A thought... Perhaps create a LARGE frame, the size that you want, from decorative picture molding. Painted or stained. Just the frame. Nail it to the wall. And then, paint the interior of it a darker color than your wall. Then....place photos that you can change inside the larger frame. They can be framed, or unframed. They could include art by your children. A sort of ever-changing gallery. But contained within a larger frame. This way you aren’t necessarily tied to A piece of art and can supply your own images.

I love big art pieces