Ah, the pleasure of forgotten toys. From Duane:
Every once in a blue moon, just dump the toybox completely upside down, every single toy out on the floor. If your kids are anything like mine they'll have a blast rediscovering assorted toys that they've forgotten about for months. All morning I heard my daughters (5 and 3) yelling "Look what I found!" and "I thought I lost this!" Works best with dolls, action figures and the like…not so much with board games.
Of course, I'm sure they'll promptly forget about them / lose them again within days, but it's fun for a quick blast when they don't know what to do with themselves. Your mileage may vary, especially if "dump the toybox completely upside down, every single toy out on the floor" is the way your kids play every day :).
Related: Rotate toys to keep them interesting