The technichal gags on Somethings Afoot have been my most enjoyable tech pieces in my CT career.  I went fairly low tech using pulleys, strings and air hoses.  My favorite was the phone gag.  I had a old telephone we rigged on a telecoping arm so it came about five feet into the room through the secret pannel.  The ear piece was connected through the phone body by a thin air hose painted black instead of telephone cable.  The ear piece was preloaded with baby powder (sometimes way too much).  When the actor answered the phone, I would shoot air through the other end of the air line causing a large cloud of powder to envelope the actor's head.