Writing. Composing music.
Doing those things, for me, encourages me to express what is inside, and as I gain experience in doing so, and become more comfortable with it, the natural desire grows to expand the expression in new ways...to try different types of writing, different forms of music that may not come naturally, so that my creativity can become flexible, and so that I feel I can express myself with more freedom.
Creative witticisms. I like using wordplay, especially multi-layered puns (puns that apply to multiple things at once, which increases the humor and response in the listener) and riddles. When you do this, you are giving a workout routine for your creative mind to find connections. Here is a visual example of a wordplay riddle I drew up for fun once. The riddle answer is one of the MBTI types...can you guess which one?
There are two kinds of creativity I think about in particular: Creating things that defy the conventional and ordinary through discovering a unique set of connections between different elements, both external and internal. And then another form of creativity is to take what is ordinary and conventional and to accentuate and exaggerate its attributes to produce a particular effect that may be clever, unsettling, melancholic, etc. It's good to exercise both, I believe. There are obviously more forms of creativity, but I use these the most.
An idea I have for maturing creativity is to increase your attentiveness to everything. Discover the things you mentally pass by, whether objects, sensory input, or ideas, and contemplate them. For example, when you are walking outdoors, you might look at the plants, and think about which species of vegetation come across as being male or female. Imagine what they would be like if they interacted. Which ones would be gruff and intimidating, and which ones would be jocular and cheerful, or shy? Such an exercise is fun, allows personal and unique impressions to be conjured up, and encourages the formation of creative connections with the external.
I'll probably think of more later.