I will usually have a roof (preferably glass for those nice sunny days) over the space, and a heater so sims can stay there when its cold. I of course put seating, lights and plants. Depending on the size, I'll place a small bookshelf or just a couple books acting as a shelf. Easel, yogamat, stereo/wallspeaker. Did I mention plants? I'll place as many as possible, as space permits.
From a house I'm working on:
https://i.imgur.com/PwnYBq8.png
Rest of the house is empty, I only finished this little corner of it a few months ago. :D
I usually make those spaces large if I know they will be used. For smaller balconies (actual balconies) I'll place a bench and a couple plants.