IMO reading is the best one.
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Learning information about more subjects gives you more potential angles to approach any given idea
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Increasing your vocabulary makes your thoughts more precise, enabling more complex and nuanced thought patterns.
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Learning new things, especially when it takes effort, requires a degree of lateral thinking to connect what you're learning to what you already know.
The second best one is some type of hobby that requires a lot of troubleshooting. So like computers, working on engines, anything. The main thing is getting your imagination and problem solving working together.