I tend to put bedding plants in mine, pansies & violas in the winter and at the moment bizzies round the edge with tagetes in the middle, you can add water retaining crystals and slow release fertilizer balls which may help, I enjoy changing them each time rather than just one plant

I agree with Pamg - these urns, as you say, are pretty shallow, so I'd treat it like a freestanding hanging basket, if you see what I mean - if its in shade, pansies, conifer and small ivies for the winter, upright bedding fuchsia in the middle with trailing lobelia and busy lizzies and maybe something like Bacopa for the summer. Fuchsias are sensitive to frost through their roots anyway, so leaving one in a container outside all winter, hardy or not, is always a risk.

Another suggestion would be hardy succulents like Sedums, Sempervivums etc. They can look really attractive not just flowers but their evergreen foliage too. They have the added advantage that they can dry out pretty much with impunity too so that fortnight away is no problem.

Thanks Bamboo, Pamg and Fractal for your helpful comments.