That's fairly simple. If we consider PC-98 as technically canonical but near irrelevant for the current state of things (which is the impression I have of it), the only real information about Yuuka's activities are that she goes around to view and enjoy flowers that are currently active/blooming in that given season from the PoFV profile, she apparently visits Reimu's shrine sometimes and allegedly purchases flowers from the village florist according to PMiSS, and her hobby is effectively rendered as taking it easy or lazing around in the third volume of SCoOW. This isn't very much information to go off of, but I find that it's perfectly in keeping with a few of her characterizations in earlier appearances. While I don't deny that depictions we would call classic by this point like Gardener Yuuka or Bakerin have their charm, since they paint her out to be a solitary but hard-working, industrious lady devoted to her work, ultimately trustworthy though a little frightening, in the end they try to make her closer to something palatable to a given person, more likeable in the general sense, and easy to justify, since, if we primarily view her as a woman, who wouldn't want a woman he likes to be dedicated and passionate in such a clear way?
I for one can't help but feel that this takes away from some of her charm. From the very start Yuuka has been more or less quite silly. PoFV tones that aspect down a little but it's still quite intact through all of her sharp interactions. Even in PC-98 where she was technically the master of a mansion, whose shape and structure is pure guesswork, she mostly seemed to be minding her business. In Seihou, after winning a match, she would occasionally just comment on how the weather is perfect for a nap.
In spite of my disagreements with certain others posters here in certain other threads, there's something distinctly girlish about Yuuka, a sort of freedom, an unbound attitude that allows her to act the way she does, both alone, in company, and before and after fights. In a way it is childish, like the attitude of someone without any real responsibilities or deep connection to outside elements that would make them invested in the typical dealings of life. This is why I think ZUN, even after all those years, and likely a few efforts to imply he doesn't want to do much with her any more, still had the good sense to put her hobby as just relaxing and taking it easy. Aside from one section in PoFV that has interesting but unexplored implications, there's no real evidence that she does anything she does because she has to or is forced to by other circumstances. Other powerful or older Youkai have pseudo-administrative positions, seek after their own goals or do whatever they please in dramatic fashion. That last one they likely took out of PC-98 Yuuka's book and her "just because" invasion of Makai. Windows Yuuka, though less outgoing, seems content with her own company. In that sense, this almost completely unaffiliated, independent position is very much properly, if abstractly, Youkai-like.
In short she likely has no other hobbies, but she might not even have hobbies, since, if you have no real work, what is there to distinguish one thing as necessary and another as volitional and done for recreation or similar?