What can I put in a large round garden?

Do you want annuals or perennials? What is you location.

How large is the circle?

If it is quite generous, how about anchoring the bed (off-center) with a multi-trunk serviceberry tree?

The birds love this tree. It has white blossoms in the spring for about a week, produce small black berries that the birds absolutely love, and then the leaves turn a beautiful red in the fall. This is a smaller tree.

Then put in a few colorful shrubs--some evergreen and a few that blossom. Fill in the remainder area with bulbs, day lilies, and perennials that you like, remembering to keep the taller ones in the center area, medium plants in the middle, and short ones near the edges. Putting in a bird bath would also add some interest. Put in 3 - 5 of each of the perennials; perhaps even more in a few areas. You want to create a splash of color.

if you want a formal look many herb gardens are done in circles with a sun dial at the center. I' also seen round gardens planted with flowers as a color wheel. bird bath or fountain for center.

I have a round garden which has a birdbath in the center. I had herbs, then flowers around it. The problem I had/have is that blue jays will bring blackberries to the birdbath to wash off. Sometimes the blackberries slipped away and now I have blackberries growing around the birdbath. I used to try to clean the berries out and plant again only to have the blackberries win. I decided to let it become a blackberry patch. It is becoming unmanageable and overgrown. Now I will cut down the blackberries put a weedblocker cloth down, cover it with rocks and put big pots of herbs around the birdbath.

I like lillies and blue bells that come back every year. Also a nice bush in the center like a peoni, hydrangea, rhododendron, and I really like my fushia cause they attract hummingbirds who will stay in your garden.

just depends on your light and weather.