AN ENGLISH-SPANISH DICTIONARY (GRANADA UNIVERSITY, SPAIN), 14.4
be up to
(v.) = hacer, tener entre manos, tramar, estar haciendo.
Ex: When I saw what he was up to, I drew back for a punch and hit him so hard on the nose that he fell on his back and lay there for some time, so that his wife stood over him and cried out 'Mercy! You've done my husband in!'.