You could make a more realistic tree like the original string tree mural, but we took inspiration from a holiday card to change up the idea. The best part is that it would only require nails or push pins at the end of each branch. If you're a renter and not into adding the holes to your wall, you could use sticky release hooks, but it won't have the same polished look (just make sure they're rated for the correct weight of your ornaments).
You'd be able to hang ornaments across each string and you could add more horizontal strings if you needed them for more space to hang extra ornaments. It's a simple idea that can be used no matter how small your space is.
Related: 10 Modern Wooden Christmas Trees