An interesting and unusual centerpiece can be made from nearly any unique object, provided that you take into account what practical characteristics make a good centerpiece. If the piece is intended for guests eating seated at the dinner table, it's important to make sure that the centerpiece isn't so tall that your guests can't see one another. It's also important that such a centerpiece be interesting and attractive from all sides. A centerpiece designed for a serving table, however, can be taller and be designed for viewing only from one side.
It's also a good idea to choose a centerpiece that is relatively food safe. Avoid any toxic flowers (such as Lily of the Valley or poinsettias) or materials that are likely to shed or deposit a lot of dirt, leaves or debris on the table area. However, in some cases a flower that sheds petals can be a good thing; a scattering of fresh rose petals can make a fabulous part of a centerpiece for a romantic dinner.
Flowers and candles make up the most common centerpiece elements. Working around these traditional pieces with more unusual vases and holders are a great way to go.
Edible centerpieces are a great way to make something exciting and fun for your guests that's unique to your menu. Food sculptures are easiest to carve from produce, like eggplant penguins or fruit salad dishes cut from a hollowed-out melon.
If you're looking for something a bit more bohemian, small sculptures make excellent centerpieces, especially when you can incorporate candles, flowers, or small dishes of things like sauces into the shape. If your sculpture won't hold water or isn't fire-friendly, you can still use artificial flowers, electric lights, or both.
Many natural objects can be used as centerpieces in much the same way. Look for large pieces of driftwood or sea shells (like conch shells) or clean, interesting rocks.
Another great way to do something with a lot of unique personality is to create an object collage. Arrange a collection of small toys and curiosity objects in something like a wicker cone, flat trundle basket or on a series of differently-tiered platforms (you can make some of these by arranging three or four overturned canisters in a clump). This is particularly good for children's parties, themed parties, or if you're going for a kitschy look. Alternately, a similar type of arrangement can be used to present small party favors for your guests to take.