The Fitness Center at Fairmont Chicago Millennium Park
The fitness center at Fairmont Chicago Millennium Park is mentioned by professionals. Out of 13 professionals that we monitored, 3 endorsed the fitness center, such as Frommer's, Oyster and Star Service.
Fairmont Chicago Millennium Park 
Frommer's
Oyster
Star Service
Frommer's "For in-house pampering, visit the tranquil spa, adjacent to a good-sized fitness center"
Oyster "But you'll have to pay extra for just about everything, including parking, the fitness center, and Wi-Fi"
Star Service "Guest are still charged a $10 daily fee to use the gym, though they will waive the fee if you sign up for Fairmont's reward program"
