Is there a word for practicing in secret to surprise or impress?
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There are a great number of words for operating secretly (clandestinely, covertly, furtively, surreptitiously, stealthily, unobserved), but none in common usage that implies a happy revelation at the end of the day. Let's face it -- skulduggery is skulduggery, and the actor is working in the hope of being undiscovered while performing the actions in question, and will act in the same odd ways that someone who was up to no good would. The language used to describe it shouldn't be too very different until one gets to the twist.
"Plotting" said with a friendly smile is the only thing that comes to mind. "Mischief" might be similarly spun as in "up to mischief".