I tried to ask my fiance what he thought the most romantic thing I had done, but he told me to figure it out.
I think the most romantic thing(s) was surprising him with dinner a couple of times. He would call me throughout the day about what a rough day he was having because of class, and then work would be unpleasant. I'd find out what time he got off, and ask him to come over. By the time he made it over, dinner would be waiting for him.
It doesn't always take some dramatic gesture to do something romantic for someone else.