This is a simple, yet incredibly useful concept. At least for me.
I keep a record of every idea I have, no matter how out there or dumb. Every character, plot device, even phrases that I think would make good titles or good punch lines. I always have pen and paper or a voice recorder handy. I type all this stuff up in a basic text editor nearly every day.
Now, granted that most of these ideas are never going to see the light of day. Even so, I have enough for several years of writing, without breaking a sweat. I pick whatever idea seems most appealing, whenever I'm not sure what to write about next.
I often return to partial ideas, and add on, as well. Many of my dumbest ideas have ended up being killers, once that last little piece of the puzzle fell into place.
The main use of this is just to constantly keep my juices flowing, to practice thinking in a creative fashion. I also get practice typing, and developing ideas beyond that first blush. And I have a massive resource for story prompts, that is growing larger every day.
These ideas can also be combined, two ideas can make a good story where one would be thin. Characters often evolve from one idea to the next, or make it in wildly different stories. Snippets show up when I least expect them, saying "Here. Here is where I go." and inserting themselves into my current work.
I like to think I am earning interest on these ideas as well. Creative interest. I'll be rich in imagination, someday.
The idea bank. Give it a try.