I had some clear glass balls I got last year on clearance that I painted mint condition, the leftover paint from the craft closet project
Add some real greenery in there to give it a fuller look. I also added several berry branches I clipped from our backyard. I wrapped the tree in the green striped ribbon. Throw in some pine cones and dried hydrangea and it's getting fuller. The point is to fill some of those gaps and give it a bit more texture.