One thing I do is keep a note app (I like Bear) on my phone. Then anytime I have a story idea, or a funny thought, or something interesting I noticed, or just a good line, I put it in a note called: Medium Ideas. Then I check the note whenever I am looking for something to write about. I pick an idea and develop it.
I usually have multiple stories going at the same time in a variety of lengths and styles (a novel, short stories, humor, etc.) If I get stuck on one, or bored, I move on to another.
Or I simply don’t write. I know I need a creative outlet in my life, so I have others beside writing. I write and record songs, play in bands, make comics, photography, etc. I switch between them depending on which is holding my interest at the moment. Then they don’t get stale. And they are fresh when you come back to them.
As you mentioned, walking is another great way to get ideas. If you don’t have music playing your mind will wander…
P.S. Thanks for the mention Alanah! I plan to do a montly list of my favorite reads.