1) Flooring. I love to add outdoor rugs or other coverings, especially when you don't have a defined porch or patio. It separates the furniture from the landscape and creates wonderful ambiance. Using a geometric rug is my preference, so you don't compete with natural elements yet retain the sense of outdoors. This one is from my shop and is available in several sizes and colors.
2) Lighting. An essential part of an outdoor room is its lighting. Whether you're having a dinner party or just conversation, good lighting makes spaces more pleasant. If you don't have receptacles to plug in string lights, which are the easiest type of portable lighting for outdoors, candles and LED candles are great. This lantern, from Crate & Barrel, is eco-friendly and has fabulous texture and clean lines. Take a look at the LED candles that you can turn on at 6, run for 5-6 hours, and they go off and cycle on again every day at the same time.
3) Planters. If your outdoor space is a patio, balcony or other space without greenery, it's really nice to add plants. The right planter is key to maintaining the plant through summer and beyond, but also adds character and style. My personal favorites are ceramic urns and vases with texture and unique scale. You can plant a tree, grow a vine (using a trellis) or simply plop in a plant. Take a look at the copper and ceramic planters and the trellis collection @ A Touch of Tradition.
4) Seating. I don't believe that all matching furniture outdoors is essential, and mix teak with iron and wicker all the time. Purchasing pieces that will LAST is the right way to go. Teak is one of my all-time favorites and looks great whether it's oiled and brown or aged and gray; it depends on the locale and the style you're after. My teak pieces are plantation grown and will last for decades.