**beginner painter** tips on how to make the background look less flat/boring?

I am //VERY// new to painting, and i’m working on this painting of my girlfriend and i to give to her for her birthday. I have all of the details that I want, and i’m scared to add more detailed things to the background (worried it’ll look bad and ruin it lol) but there’s so much empty space and i want to make it look more full/add more depth.

Does anyone have any advice for a beginner as to how I can make it less flat/empty?? thanks!