Gym memberships are popular with nomads since they provide one of the best places to shower for those who don't have a shower in their vehicle or want an unlimited-water shower. In addition to showers and workouts, gyms generally also provide free internet access and a place to recharge devices, and some may allow overnight parking with permission of the manager.
Ideally you will be able to access a gym associated with your workplace or school for free; be sure to ask if one is available. If not you can get a subscription to a commercial gym like Planet Fitness, Anytime Fitness, 24 Hour Fitness, etc. In some areas the local rec center or YMCA may be cheaper.