Flowers are always a good idea. Orchids, lilies, and delphinium are all nice and tall, and grasses and bamboo are very in right now. I also like wine bottles with candles in them. I have seen tall clear vases filled with fruit--limes make a strong impression.
Try striking, tall flowers, such as the bird-of-paradise, simply or in an arrangement; consider a simple vase with lucky bamboo; try a tall pillar candle or grouping.
Slender branches from your yard can provide height to a lower arrangement. An intersting paper or colored foil twisted around dowels or a handful of sticks can be fun and simlpe. Cover craft styrofom balls with fabric and stack, using toothpicks as anchors. Use elements from your job or hobby for inspiration.