Exercising effectively outside of a gym can appear daunting, partly because somewhere like a park, for example, does not contain the automatic fall-back of rows of weights and equipment which can be worked through (almost mechanically if this is your habit) as in a gym environment. But that does not mean you cannot get a solid workout when exercising outdoors.
The key to exercising effectively in the open air is to target all the major muscle groups when doing so and to ensure that each group is exercised in your workout. This can be achieved with free weights, through resistance against fixed objects, or by using the body as resistance (the press-up is just such a resistance exercise that uses the body in this way).
But the key is to plan the workout first, as in this example with Andrew here, using resistance bands that target different major muscle groups so that, over a short time, each group is exercised effectively.