The gym at Sunday Schwielowsee Resort - Apartment appears to be an area of concern for guests, consistently highlighted for its small size and outdated equipment. Visitors often find the fitness room too cramped for the hotel's capacity, and several reviews mention disappointment with malfunctioning machines, including an absence or limited function of popular equipment like treadmills. The need for modernization is emphasized, as the current state of the gym does not meet guests' expectations and is described as unkempt. Though some reviews touch on the wellness and spa areas as being inviting and adequately clean, the gym facilities are seen as an aspect in dire need of improvement to enhance the overall guest experience.