Floating Flower Tip - Most flowers float pretty well on their own until they start to pool with water and slowly sink. For a longer lasting centerpiece, use bubble wrap to make little skirts for your flowers.
For one of your table options grouping floating candle vases together is a pretty option for a non-floral centerpieces. To add the "wow", tint the water in some of the vases a dark purple and some a light purple and float gold candles.
DIY: How to Make Evergreen Topiaries - tutorial shows how to make these topiaries using evergreen branches, greenery and pots. If you use craft store greenery, you could use them from year to year - via Inspire Me Today