Decide how you will use the island. Before getting the seats, it is important to know how you and your family plan to use the island. If you love entertaining guests who come to your home, you may only need stools for a more casual setting. If you have children and they are going to use the island to do their homework, then you need a lower counter and seating area.
Islands can "float" as well. Smaller islands used primarily as an aid in cooking or baking can be fitted with wheels and moved around the kitchen as needed to aid in cooking. Whether you consider a floating island will probably depend on what you have installed on the floor now, or plan to install (a soft wood will be nicked and scratched).
Pick a countertop for your island that matches your other counters or make it stand out by using a contrasting material. Also, your choice of cabinetry and decorative elements can make your island into a real quality item, similar to a fine furniture piece.
If you have a big island that can accommodate many people, consider using full benches. The benches can be with or without backing to allow for more comfortable seating.
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