The ‘Feynman technique’ for actively digesting, understanding and remembering a topic of interest involves reading/researching, then explaining (as if to someone else) the key points of what you’re trying to learn. This highlights gaps in your supposed knowledge, and probably also tricks the brain into assigning a higher importance to it, thus enabling better recall.
(Reading is easy. Try reading a chapter and then summarising it in this way!)
There is a huge difference between “play” and “practice”, although it seems to me that both are necessary and that deliberate practice can only be sustained if it becomes a kind of intense play. “10,000 hour expert” doesn’t mean 10,000 hours of play, unfortunately.