I spent about four years living in a third floor flat, with a balcony of a similar size. My lessons learned are:
Things dry out. Of course that's a general issue with container gardening, but our balcony was sunny, and often really windy too, and in hot weather the walls and floor got baked. So if your balcony is going to get the sun, I would recommend glazed pots to retain moisture, and unless you are at home enough to pop out and water things often, choose your plants for drought-tolerance.
Ants like to make nests in pots, particularly if the soil is dry. And being on the third floor doesn't stop them. I only realised this when they started marching into the flat.
This may not be an issue for you depending on the size/layout of your flat, but we didn't have many places to store things, and had glazed doors and a huge window to the balcony. So whereas in the garden you could move pots around to enjoy things in flower and hide away anything that wasn't looking so good, on the balcony it was just kind of there all the time. So worth considering how things are going to look all year round, and make sure there is enough still looking presentable in the winter. If there is nowhere in the flat to store pots/compost/tools etc., then maybe a small outdoor storage box would help to keep it tidy, and could double as a place to stand a plant pot, or a glass of wine. (Though I understand the potential safety issue with climbing DCs!)
As you asked for inspiration, I'm trying to think what actually grew well on our balcony. Herbs would be an obvious choice. Cosmos also did really well, and we grew some small sunflowers with the DCs.