Pillows and Window Treatments are fantastic because they make it possible to add color, pattern and a POP to any space in your home, typically without spending a fortune.
You can also DIY throw pillows and curtains without too much effort. Often you don't even need a sewing machine!
I've made throw pillows out of dollar store bandanas, and my one-minute bolster pillow covers are great for seasonal decorating updates. And yes, it literally takes one minute to make them so, why not?
Outdoor curtains (and you can DIY outdoor curtain rods, too) elevate any space - it always feels slightly exotic to know you can curtain off a space, even if it's in your own backyard.