Go on and break a sweat. FLEX Self Magazine Workout in the Park What: Outdoor group fitness classes are held alongside beauty consultations, nutrition seminars, giveaways, and more. Why: There’s strength in numbers. When: Sat., 11 a.m.-3 p.m. Where: Butler Field, Grant Park, 350 E. Monroe St., at Michigan Ave. Tickets online at evolutioneventsonline.com. CHILL La-Tea-Spa Promo What: Take a friend to Four Seasons Hotel Chicago and score half off a spa treatment and tea service. Why: The Hawaiian honey body wrap pays off. When: Thru Sept. 7. Mon.-Thurs., 8 a.m.-8 p.m. (Tea served 3-5 p.m.) Where: 120 E. Delaware Pl., at Michigan Ave. Reservations (required) at 312-649-2340. TRADE Open Books and Gapers Block Book Swap What: Trade gently used reads with fellow bookworms. Why: Leftover books fund literacy incentives. When: Thurs., 6-9 p.m. Where: Black Rock Pub & Kitchen, 3614 N. Damen Ave., at Addison St. (773-348-4044). EAT Market What: Just-opened bar/resto dishes American classics (burgers, pasta, fish) in a massive beer garden and on a rooftop patio. Why: Nothing beats market value. When: Sun.-Fri., 11 a.m.-2 a.m.; Sat., 11 a.m.-3 a.m. Where: 1113 W. Randolph St., b/t May & Aberdeen Sts. (312-929-4787). SHOP Helen Yi Vintage Clothing and Jewelry Trunk Show What: Throwback clothes and accessories from major designers (Chanel, Lanvin, Halston, others). Why: It looks better with age. When: Thurs. & Fri., noon-6 p.m. Where: 1645 N. Damen Ave., b/t Wabansia & North Aves. (773-252-3838).