The few time a gold coin hits the plate, it bounces off. If he keeps trying, he could probably get one to stay eventually (sort of like those games you play at a fair)
After listening at the door, you open it onto a simple 20 x 30 room, 30' high, and the extra 10' in width to the right. There is a door on the east wall at the very top of the room. The room itself is swept clean, except there is a gold dinner plate lying on the floor in the exact centre of the room
The elevator descends, and clanks to a stop. You open the door, weapons ready, and your light spills not into an empty rom, but reflects off a giant wall of clear jelly!