When you first come to downtown Cancun, you'll probably be driving, walking or taking buses on the main thoroughfares, getting an impression of too much traffic and too much concrete - although there are green islands in the middle of many intersections and all the roundabouts, usually with beautiful flowering shrubs and trees. But there's more. When you want to explore, or when your destination is in walking distance, leave the main avenues on any side street, and behind many condos and commercial buildings you'll find neighborhood parks that make for a much nicer walk, with paved paths (yes, cracked and broken from tree roots, but still, cement paths) and in some cases, playground AND exercise equipment.
There are far more such stretches of grass than Google maps show. In the center of town, there's a long park area between Bonampak and Ave Tulum in the section just north of Plaza las Americas. From either avenue, just walk down any street : Tierra, Lluvia, Agua, Fuego - and follow the paths to a delightful stretch of green, that Google doesn't even hint at.
Heading north from there, between Bonampak and Nader there are a number of small, shady park spaces.
Some of these neighborhood parks are recognized by name. Parque Gaia is an extensive park, mostly shady, between Acanceh and Circuito Copan - that is, just a couple of blocks west of Plaza las Americas.
I don't know the name of the park that runs for about five blocks from calles Reno and Venado on the north (Just south of Xcaret / behind Office Depot) to close to the southern curve of Labna - all just a block west of Ave Tulum, but I cut through there instead of walking along Av Tulum. You'll nearly always encounter joggers and dog-walkers in this pleasant neighborhood. Some parks are (nearly) litter free and posted to try to keep them that way; others get no respect at all; but I still find these routes to be a great way to walk through town, away from traffic.
I need to pinpoint more locations, but near Mercado 23 there is good exercise park. Other neighborhood parks with exercise equipment are in SM 45, east of Kohunlich, and Region 518, Parque Barrio Maya and Parque Terra Nova. I love these facilities: the equipment is sturdy and simple, as you use your own body weight to create a workout. Fun, free, outdoors, how can you beat that?