A popular centerpiece is to use cylinder vases with a floating candle in the top. It is worth buying bigger floating candles for this style as small ones end up floating to the side and won't stay in the middle of the water.
You will also need to think about the height of the vase and what else you will end up filling it with. And by that I mean you don't just want a vase and a candle on the top as there is too much empty space in the vase.
This is why you will see flowers or leaves submerged in the vase as it gives a better look overall. Stick with flowers with thick stems - roses, tulips, carnations and orchids do really well. The great thing about submerging flowers is that the water magnifies them to make it look like there are bigger flowers in the vase. That way you don't have to use as many flowers!
You can have pebbles at the base of the vase if you need to hide how you secured the flowers to stop them floating.
Photo by Wait Scm
You can secure the flower to the base with aquarium glue, or a frog clip, fishing wire and weights (that's if they are really tall vases and a heavy flower!). My personal favorite is to wrap florist wire to the stem and then wire a pebble to it, when you place the stem in the water you can then cover the wire with more of the same pebbles.
Photo by DR Ranch
If you want to add more excitement to the vase you can add extra light with LEDs placed amongst the pebbles at the bottom. The light from these will shine up and light the flowers from below. This looks stunning at an evening reception.
Glass marbles are a good way to add extra color to the centerpiece if you didn't want to use pebbles and one very fun way to add a whole load of color is to use water beads. The water beads you add to the vase along with your flower and then you have a stunning way to hold the flower in place. The beads come in different colors.
Photo by Championship Catering
If you have long tables at your reception you could have the vases along the table. Or to save money you could have one large cylider vase in the middle of the table and then have glass votives along the table.
A round table could be one very big and tall cylinder vase or a group of three vases in the middle. Again if the table is big and your main centerpiece is being lost on the table you could look to increase the size by extending the centerpiece out. You can do this with; petals, pebbles, table runners, and more candles.