From my experience, it can feel awkward, uncomfortable, and even unpleasant, for adults to 'play'. But I think that's because so many of us associate 'play' with that first definition: to engage in activity. There's a belief that, in order to play, we need to be doing something, playing something - a 'something' is often associated with a time or an experience that we no longer like or never enjoyed in the first place.