You must pay 500 PD for a ticket for a tour of the Skyscraper. A guide will lead you up the Skyscraper, and there you may encounter trainers and/or events. There will be some sort of surprise or award if you make it to the top. There will be a few Mini-PokeCenters to heal your Pokemon.

Please note that this is optional. ^-^ You have something else to do here, too - the gym!