Any annuals would work. Also groundcovers that might have a long drapey habit - fancy leaf ivy, vinca, lysimachia, etc.

Are you looking to block sight lines, and if so, just in summer or year round? If year round, where are you and what USDA growing zone are you in?

Depending on whether you are in a cold zone like me, it might make sense to plant a small tree or large shrub either in front of or behind the planter to block sight lines, and then use the planter for seasonal color, using annuals in summer and cut evergreens, etc for the winter months. Where I am, a planter like that would freeze solid and not many plants would be able to survive. If you are in an area where you don’t have to worry about the planter freezing solid, you will have more flexibility.

NHBabs I am trying to block sight lines year round. The tree that’s on the front side I will cut down soon but haven’t yet because then this space would be even more open. I live in NC.

I might try an evergreen vine on a trellis there, or as I said above, plant a small tree or tall shrub in the bed next to the planter along with seasonal plantings for the planter. There appears to be a fairly limited root area in the planter for something large enough to block sight lines year round such as shrubs. Or alternatively, install along the back edge of the planter a matching short fence that goes from the top of the planter to the same height as the top of your current fence and then put in seasonal plantings as illustrated above to suit your taste to break up the look of solid fencing.