Another easy thing to do to make it a little sharper is to use different line thicknesses (known as line hierarchy) on different items. They should be black with finer tipped pens for finer lines and broader pens or felt tips for fatter lines. Buildings tend to look best with a very fat line, eges of walks with a nedium, and most plants with a fine line, but different plants look better with different line thicknesses. A coarse leafed plant like a rhododendron could be with a fatter line while a more delicate plant could be an ultra-thin line.
The light, shade, and shadow give you a third dimension, the line hierarchy makes it crisp. These techniques work regardless of your drawing talent.