Grab, neutral air if people don't hold down, stomp and knee if the opponent doesn't see the start up and shield or avoid the hit, uair if you're super low or the opponent is in the air for some reason... So yeah, the closer to a real neutral situation, the less you can startle your oppnent, and the better they are, the more it moves just towards grab, but you can try other things with tricky movement or rapidly being able to change pace, and mixing low uair with low single hit nair can work if you manage to stay at good spacings when lower. You can pretty much do whatever to punish a lot of attacks or other laggy things though.
Obviously, more options become available at higher percents. Uair and single hit nair tend to work better vs. floatier characters, as would gentleman to start a combo. Dtilt can work better as a combo starter (or extender) if the opponent lands on it, and vs. fast fallers, though it's a little slow to just do in the neutral outside of whiff punishing.