Keep it simple, which is a good decorating rule of thumb in any small place. Beyond that, it depends on what you're looking for. Do you like plants? I'd hang some from the roof, if you have a roof, and that would leave ground space for seating and/or a table. If you don't have a roof you can put some smaller planters, or maybe just a pot in each corner. The plants you choose, especially if they are on the ground, shouldn't be anything bushy that will take over the space. Roses are good, but you'll need to keep it cut back. Other perennials are good. The main thing to keep in mind with outside planters is that you need to replace nutrients into the soil yearly so that the plants don't suck all the good stuff (nitrogen, for example) out of the soil and then die.