Luckily, getting fit doesn't require a big field or expensive equipment. I do push-ups and sit-ups every day in my room. I do pull-ups at the jungle gym, or even on scaffolding. I run outside when the weather is nice, and when I can't, I run the stairs in my building. Some days I mix it up with a jump rope and medicine ball in Central Park.
The best thing about these workouts is that the only person you're competing against is yourself. If yesterday I could do one rep, then today I can stretch myself by going for two.