Think about height levels in terms of how they relate to the adult human body, which affects how they are viewed and experienced. Anything below knee height is viewed from above. Waist-high elements are seen at an angle, and form a screen, partly blocking views of anything immediately behind them. At shoulder and head height, dense or opaque elements (such as closely planted tall shrubs, hedging, or high screens) will completely block a view. Structures above head height, for example a tree canopy, can create a sense of seclusion.
Few built elements can compete with a mature tree for interest and drama. If you don't have mature trees you can create fixed elements using hard landscaping, then increase interest by combining plants of different heights.