Jungfraujoch Plateau is an open-air snowy plateau, where you can walk around on the glacier snow. It has fantastic views of the glacier and surrounding peaks.
To get here, follow signs inside the Sphinx Observatory to the "Plateau", which requires riding an elevator to the lower floor that gives access to the snow.
When you exit the visitor center onto the snow, you have to walk uphill for a minute to reach the plateau. The snow can be very slippery and difficult to walk on. I've seen lots of people slip and fall on this part. If anyone in your group has limited mobility or difficulty walking, this may not be the best option for them.
It can be very cold, even in summer. So wear warm layers and waterproof shoes. You may also want gloves and a warm beanie hat or scarf. It's also very bright - sunglasses are definitely recommended.
Make sure to wait in line to take your pic with the Swiss flag - it's iconic!