Can you have some pieces of stone cut just like you would for a kitchen countertop? Or could someone make concrete countertops like they do in kitchens? Then you could plants in pots on them in pretty weather.

Good ideas above. Or you can fill the empty planting areas with gravel.

A wood bench for sitting and/or flower pots.

Cut 4x4 lumber the width of the planter (15 inches looks like?) and place the pieces side by side on top, filling the entire length. Secure them to the brick with construction adhesive. Sand and stain a rich wood tone.

If the bench is wider than the planter, have it flush in front so no knee bangers.