Some nice flowers are a good housewarming gift and I would enjoy getting flowers as a housewarming gift.
Other things that make a good housewarming gift or gifts are wine, house warming cards, champagne, welcome mats, placemats, and gift baskets or fruit baskets.
Years ago people used to bring Salt and Bread to housewarmings and even a broom.
But most people nowadays would likely be offended if you brought them a broom since it means cleaning and they could see it as an insult to them and they may think you're telling them to clean.
Although the Salt and Bread is still a good idea even though it's mostly a Jewish Tradition anybody could really bring Bread and Salt over to a housewarming.