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.
Mason jars as drink glasses for the evening and they are a simple keep sake. Guests could fill at the end of the night with favors of choice ( candles, candy.etc ) I LOVE this idea! I'm sure it would be pretty cheap too!