I'm unsure about this too, so any advice would help.
Solo cruiser, sailing from UK, crew appreciation already included in fare.
I'm very low maintenance, once a day is enough for my cabin refresh. I don't need turndown, I don't drink - so no bar staff on my radar. I eat in the MDR, but don't choose any fancy alternatives. If I want tea/coffee or a snack during the day I go to the buffet.
So what ( or more to the point who) would I be to giving little cash envelopes to?