I think sly fits the bill. The third dictionary.com definition defines it as
playfully artful, mischievous, or roguish
And, in fact, since (according to the comments on your question) you're referring to Richard Feynman, it might be helpful to mention that a number of examples can be found in the wild of people using this exact term to describe Feynman (this is clearly not a scientific survey by any means, but just a collection of examples).
For example, in the introduction to QED A. Zee writes
I chuckled a few times as Feynman got in some sly digs at other
In a forbes.com article the author refers in passing to
a bit of Richard Feynman sly magic
In a review of a comic featuring Feynman, the reviewer writes
The cover alone almost feels more real than photographs of the man,
having boiled down his essence into this inquisitive, sly smirk, the
outward representation of the scientific curiosity that drove him
every day of his life.
In a scientific paper building upon some of Feynman's work, the author writes
This result came out eight years after Feynman's death, but I can
imagine him looking at the equation with a sly smile.
And that's just some of them. A google search reveals a number of other examples.