I put in a bench or some sort of outdoor furniture (like an item from the Perfect Patio SP) then I have a bookcase near the door leading to the balcony so hopefully Sims will go outside to read. With NK, I'll use a rocking chair out there; Sims are quite drawn to those, then in inclement weather seasons (depending on the world) I'll move the chair inside. (I know some CC makers have done rocking chairs that don't look so "country" that work for more modern builds, but I'm trying to resist CC till I know the game won't get more updates.)
Like others I also put hobby/skill items like easels, workbenches, yoga mats/meditation stools, and such on the balcony, especially if my adult Sims will be the only one using them (if they're childless or their kids are too young to use the items themselves). Decor like plants also help to fill out the space.