Trailing geraniums, trailing nasturtiums, dwarfing cherry tomatoes such as the Balcony series, could even try strawberries but could be using lot of space for not much reward

It looks quite small, and as if it's on a wall, so something that trails would be nice. You can get small trailing plants such as Lobelia, Alyssum, or a trailing Fuchsia or Pelargonium.
Some crocuses or miniature daffodils would be colourful for next spring :)

Is it on a sunny or shady wall?

any dwarf bulbs that are available in gc will look pretty. you will obviously need a liner to stop the soil falling out.

You might want to consider small heathers at this time of year then in the very late spring go for trailing plants as suggested.

Thanks everyone it's given me a starting point it's quite a small bike planter but I appreciate the suggestion you have made

I think this is a case where 'less is more'. I would keep it simple - maybe a couple small potted plants, but nothing that would overwhelm the design. There are several good choices above. Trailing begonias come in all colors & styles.

Did you know you can get fibre liners for a like this?
I too would go for something semi succulent such as pelargoniums (geraniums) as they do best in dryish compost.