Soft corals, zoanthids, mushrooms. Leathers, if you decide to add clownfish you can put in an anemone, you can add some Lps coral like hammers, torches, and frogspawn, I'd recommend if you want to add any sps keep your calcium as high as it can be and have strong waterflow with strong intense lighting. But for a biocube I'd remain with mushrooms and zoanthids. Mostly the soft corals especially if you don't have intense lighting.