Once you have a gallery wall set (click here for step by step how to create one) you can keep things interesting by switching out any of the photos/art inserts inside your frames! I first decided to do this for the holidays and give my gallery wall a Christmas makeover. It was a very inexpensive and easy update that makes such a big impact.
I switched out my four largest frames, which are 20"x24" floating wood gallery frames, the photos inside are all different but they are each the same size, 11"x14" and of the same style to keep the wall looking cohesive. These vintage photos I found online, saved them to my phone and then ordered them in the size I wanted off my Shutterfly app. It was only about $7 per print and although I might have been able to get nicer prints off Etsy, from target or somewhere else, these were cheap and easy which was my priority.
I also switched out two 8"x10" champagne colored frames that I have, I decided to use my own resources and paint simple yet festive peace signs on thin canvas. I painted one red and one green to keep with the 'Traditional Christmas' theme. Since the paintings are on canvas I did need to remove the glass from the frame to allow the backing to fit, but I like how the canvas and paint bring in more texture to the the space, where the glass would hide that creating a cleaner more modern look.
If my budget had allowed, or maybe after a few years of collecting, it would be nice to switch out every insert/piece on my gallery wall for the holidays. A wooden Christmas tree sign would look great right in the center to replace the hemisphere print.