you will have to put drainage holes in it before you plant anything in it.
think about some trailing plants such as fuchsia, begonia might work not sure if the trailing ones need more sun though. These would soften the edges. Then ferns would look good as would some of the carpeting plants like Ajuga that come in a range of leaf colours.
are you thinking more perennial or annual flowers?

How about as a raised pond water feature? Lots of water plants to choose from, including pigmy lilies and if you make landing stages of pebbles / rocks etc bound to attract all sorts of wild life

water lilies don't do well in semi shade. my first thought was a water feature but the aspect put me off that idea.

Thanks Sbg and Grandad. I agree it would be suitable for a pond but I already have one, also I have already drilled holes through.
I want perennials really because I'm trying to reduce labour but might pop one or two annuals in to add a spot of colour.
I'd like something compact but with a bit of height at the back and smaller perennials in front with something to drape over the sides. Maybe fuchsias will do that Sbg, but something evergreen would suit me better. I have had a couple of ornamental ivies in basket arrangements but I'm worried that they would just go mad and swamp everything else if given the chance.

How about an assortment of veggies that grow well in partial shade such as carrots, potatoes, spinach..herbs etc sort of a mini raised veggie bed....not too familiar with the assortment which would grow well in it but surely other members would have better ideas.

I have a procumbent Rosemary and that drapes nicely and is evergreen and bonus edible :o)