Although there are a lot of things to do in Celadon City, staying at the hotel is not one of them nor is it part of the gameplay. Celadon City is the biggest city in FireRed and, along with the hotel, the city also has a restaurant, a mansion and a casino.
Since Celadon City is a huge place, it will take a while to explore it to obtain important items and Pokemon species. The player will face several trainers with the trainer Erika as the leader. She has three Grass-type Pokemon and beating her will earn the player the Rainbow Badge. Giovanni is the Boss of Celadon City. After beating him, he will leave behind a Silph Scope, which can be used to detect Ghost-type Pokemon species.