I take my clients to local parks, especially the those that have stairs and hills. Also, I do a lot of my classes on the sand as well . There are many parks around here that include all of the above. Some days we concentrate only on stairs or hills or sand and other days we combine some or all 3 in the workout. Running is also a big part of my classes.
I also like to utilize playgrounds, because there are so many things you can do there (pushups, pullups, jumps, sprints, etc.).