For example, Milo Bryant, a performance coach in San Diego, uses an exercise grab bag for his group classes. “They’ll draw an exercise from one bag and a rep count from the other and whatever comes up, that’s what they do,” he said.

Apps like Zombies, Run! — a cross between a fitness tracker and an episode of “The Last of Us” — take this to a new level. Like most running apps, it allows you to track your route and pace. The twist is how it pipes “missions” through your headphones as you run, directing you to sprint to avoid a zombie or to pick up supplies to build a virtual shelter.

The app Rouvy connects to a smart trainer, which converts your regular bike into a stationary one, for a virtual ride through different city streets around the world. It can even tweak your bike’s resistance as you encounter dips and hills. Pam Moore, a cycling instructor in Boulder, Colo., said she once biked through Beverly Hills with a friend in Portland, Ore., without leaving home.

“Although she was ahead of me, we could still ride together,” Ms. Moore said.

Our brain also loves things that seem tailored for us. In a recent study, athletes who believed they had received a customized workout plan outperformed those who thought they were following a generic one.

Personal trainers are a natural way to make use of this perception. Or you can use an app like Stronger by the Day, in which trainers take your fitness stats (the heaviest load you can lift, for example) and produce a strength-training program adapted for you.