External filter, heater, more lighting, Co2, more fish, new substrate, plants or more plants, stand, lid, Ro unit, UV filter, more dechlorinator, heater guard, digital thermometer, reflectors, new tank, liquid fertilizer, bog wood, rock work, amano shrimp, co2 diffuser, air pump, sunken galleon, gravel filter, fish food, pH probe, temple of Rome.
Or you could just pop into your LFS and see what takes your fancy.
Sam