* They encourage walking by having sidewalks and mixed-use paths everywhere. People are explicitly taught the etiquette and safety skills for using them. In some areas, defunct railways have been remade into trails for walking, biking, and/or other activities that run through a town or between towns.
* There are plenty of benches to enable less-mobile people to walk as much as they can and support prosocial behavior.
* Neighborhoods frequently establish walking routes with signs and color-coded markers to tell people the routes, distance, and points of interest. Most of these also offer guided tours at certain times.
* Cities are more walkable in general. This has many benefits. Compare the walk scores of different cities.
* Malls, museums, fitness centers, and other large public buildings often encourage walking by posting routes, distances, and times of lowest traffic. Mall walkers in particular are a mainstay of T-America's thriving mall culture.
* Parking for cars is often kept to large lots or structures at the edges of shopping neighborhoods. Buses and shuttles tend to drop people at one end and pick them up at the other, using transit hubs with nice amenities such as shelters and toilets. Pull-throughs, dedicated parking spots, shuttles and other services provide support for people with disabilities. Some towns have car-free areas.
* Traffic calming measures reduce risks and diminish the dominance of cars in public space. This supports all kinds of human-powered locomotion.
* There are bike lanes in most towns too. Bike-friendly towns often have portable corrals where you can park 10 bikes instead of 1 car. \o/ EDIT 3/7/18: For a discussion of poor bikers, see the stoker comment below.
* Skateable sculptures invite people to navigate them with a skateboard or roller skates. You can also climb on them. Many parks and plazas have at least one such feature. Benches and walls can be done in this mode also, which makes them very sturdy so they wear well over time.
* Parks often include one or more sets of outdoor exercise equipment. The simple ones for calisthenics are the most common, but nicer parks have moving equipment too. These customarily include a sign of suggested exercise routines, and sometimes the calisthetic ones also do.
* There are parks of all sizes everywhere. Most towns have at least one huge city park, a medium-size park for each neighborhood, and pocket parks every few blocks. There may be specialty parks such as skateparks, water parks, golf courses, etc. Good parks customarily invite a wide range of physical activities, often with clubs or other regular events for added encouragement.
* Blacktop playgrounds are routinely embellished with painted games of various kinds or abstract uniquities to invite more activity. This entices people to explore them to see the variety as they travel around.
* Community centers, fitness centers, and athletic parks may be private or public. Most towns have at least one public place where people can go for physical activity that offers many free or cheap options. National, state, and/or municipal funds may contribute support. T-America chooses to subsidize these heavily so that more people can use them, because that makes citizens healthier, which lowers the cost of health care and improves quality of life for everyone. Even places that usually charge a fee tend to have some free days.
* Parks, trails, and other places customarily provide amenities including toilets, shelters, and water fountains with a bottle-filling spigot. Healthy snacks include both high-energy and low-calorie options. You can get junk food if you really want it, but you'll have to work harder to find it. The more convenient choices tend to be healthier.
* Young people are taught the skills of public navigation in stages. School-age children begin with point-to-point travel between known destinations and people, then move to free-range travel within their own block. Many still walk to school if they live close enough, and neighborhoods may organize adults to keep an eye out during peak transit times. Learning to ride a bike gives young people wider access to their neighborhood. When they can pass a test demonstrating basic knowledge of the bus system, they can get their own travel pass and no longer require adult supervision. Most tweens can bike or bus around town, and in fact if they don't by that age, it starts to worry people. Thus by the time they reach their mid-teens and learn to drive a car, most people already have fluent navigation skills and enough social expertise to handle themselves without close supervision for some hours.
Add that up, and you can see why mental and physical health is generally better in T-America than in L-America. Happily, these things are replicable here, just not common. We can increase them. We really should work on that.