Check the rooftop for uneven spots. Repair major flaws with leveling compound or conceal them with plants, decking or furniture. Deck the entire roof or create a smaller deck area and naturalize the rest. Tile an area and seal it with waterproof grout if you don't want to add decking.
Install lighting. Put uplights in plants and focus spotlights on features you want to highlight. Provide sufficient lighting for the cooking area if you barbecue on the terrace at night.
Add patio heaters, a fire bowl or a chiminea for chilly nights. Provide for pest control with commercially available products or plant insect-repelling foliage such as lavender or marigolds.
Place a small water feature on the terrace. Remember that water is heavy so don't do anything large unless you're certain the roof will support it. A stone basin set on the roof with aquatic plants adds interest.
Screen the roof from neighbors with simple lattices. Attach 2x2-inch lumber that is 2-feet taller than the lattice to each side of the lattice. Stick the ends of the lumber into flower pots that are at least 1-foot deep, fill them with dirt and train climbing vines on the lattice.
Add color to the terrace by installing a sun sail or canvas canopy. Affix grommets to the corners of a piece of heavy fabric and attach screw eyes to the side of the building. Slide the grommets over the screw eyes.