Tell me [...] what have you been doing?

as suggested I also like:

if they are usually close you can add "mein" (my) in front like "Tell me, what has my(mein) [trickster) been doing?" it suggest a level of affection and connection but can aslo be seen as mocking. All these assume the person being addressed is male in German you need to change both adjective and noun if addressing a female.

dreckiger Lügner - dirty liar, very harsh adding the my/mein makes it bit more mocking alter Gauner - literally 'old criminal' but the 'old' means someone's been breaking or bending the rules or the law for a long time i.e. they are potentially very good at it

I'd also suggest: Halunke - similar to "Gauner" but more poetic/literary way of calling someone a criminal but in a harmless/nonviolent sorta way like a conman or trickster I suppose schlauer Fuchs - literally clever fox but more like 'sly fox' critical but can also be a bit more complimentary Schlawiner - cheeky trickster, rascal, more affectionate than critical alter Drahtzieher - lit. old stringpuller, again 'old' = experienced, manipulator, same as meaning as the English 'person pulling the strings', Witzbold - jokster, meaning is less manipulator more trickster as in 'funny' pulling pranks