Your challenge begins with two tasks.  When both are completed, together they tell you where to go next. 
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The first is a jigsaw puzzle.  It has a missing piece.  Figure out what words are written on the back of the missing piece. 
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The second is a medal lockbox with a three digit combination.  The number of items in the cup with the peas and Post-its is the combination of the lock. But get into the box any way you like other than prying it open. 
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Good luck!