This is what it looks like atm.

I think it's beautiful as it is.

The top os the most beautiful part. Don’t cover it with anything or put an umbrella in it. Use it as a piece of scultpure. If this were a statue, how would you plant around it? That’s the task at hand.

Can you show larger shots of your whole garden so we have a sense of what kind of plant design would compliment your garden as a whole?

Depending on you lay out, I’d consider digging a square around the bricks circle, planting the square and leaving a path at the bottom of the square to walk in to the center stone piece so people can look at it up close - stone wheel inside a brick circle inside a square garden.

If it's stone, I'd probably put a bird bath on it.

I’ve seen people hookup recirculating bubbling fountains in old millstones. They can look very pretty in the right setting and add interest and sound to the garden. You could try to rig something up with the “table” base, or set it in the ground with the fountain equipment beneath it.