Boston’s biggest free fitness series is in full swing for its 11th season, with back-to-back evening classes at 5:30 and 6:30 p.m. Monday through Thursday on Seaport Common. Rotating formats span yoga, HIIT, Pilates, strength and dance cardio, with beats from DJ Sprino and a hydration station on site. No registration required – just show up with a mat and water bottle.