Going for a walk in the park does not really meet the classical definition of meditation although you may get some of the same benefits. Some of the similarities between going for a walk in the park and meditating are that they both may provide you with a sense of calm, rejuvenation, and connectedness.
The main difference is that going for a walk in the park implies that you are getting benefits from external sources. Meditation is more about getting benefits from inside yourself. For example, you may feel more connected to world after a walk in the park but you may feel more connected to yourself after meditation.