I like the quirky. The unexpected. The anomalies. Fun facts were made for me, and they spice up any novel I read. Often that means that I spend long hours hunting down details online, and chasing down the truth and its context.
I suspect many readers love to stumble across oddities, corrections to misperceptions, and bits of overlooked reality. Putting them into your writing can add spice and build trust with readers.
Many people run across the peculiar without any effort, just as some people naturally see life differently and are born humorists. For other people, it's an occupational hazard. Visual artists look for details and reframe reality. Scientists observe the world around them with an array of tools and techniques. Historians visit strange worlds on a daily basis.
If you don't have these advantages, don't despair. You can still collect a treasure chest of gems for later use. Develop your own approach, or try these:
Find surprises -- Build your powers of observation intentionally. One exercise is to look around you, right now, and find something you never noticed or you can't explain. If you are sitting in a familiar space, this might be difficult, but it's not impossible. It doesn't need to be a thing, it can be the shape of a shadow, the pattern of dust on a tabletop, or the sound of a motor.
Or, you can make it easy on yourself by sitting in a natural setting or a park. I have a bird feeder, and I've gone from identifying its visitors to discovering new things about the behavior of the different species that stop by. Sitting in a train station, the food court at a mall, or the stands of a sporting event can provide surprises, too. People will amaze you, if you actively pay attention.
And don't hesitate to use all your senses. Close your eyes from time to time and see what you discover. The important thing is to find one notable observation each day.
Collect details -- In addition to writing down surprises (in full sentences), don't forget to makes notes on things that catch your interest that you overhear, find in Web searches, or come in your wanderings. Write down questions, insights, and whatever engages your sense of wonder.
These may have nothing to do with current projects. You may never use them in a story. But they will be a treasure chest to draw upon (often years later) as you write. And you'll discover odd connections that will inspire you. To make these even more effective, develop a way to sort these so you can find them easily. As a start, try animal, vegetable, mineral, behavior, history, patterns, and behaviors.
Pursue questions and ideas -- If you want to enrich your collection, learn a little more about a nugget that fascinates you. Find an article or an opinion you can attach to it. Even better, mention it to other people and see how they react. If you tell friends Napoleon was average height, some will be delighted to learn this, and others will declare war. For a few, this will trigger an idea that surprised them. Add this to your collection.
Reflect on what it means -- Everything you collect has something to say about you and what matters to you in the world. Otherwise, why would you have selected it? See if you can figure out why it appealed to you. As an advanced exercise, try to connect the dots of several observations.
Apply judiciously -- Okay, here's the tough part. Don't take a shovel full of fun facts and dump them into your manuscript. This will drive your readers crazy and kill your story. (I've seen this happen most often in historical novels and science fiction. These writers just can't bear not to share. Too much information -- indeed.) Be selective. Make sure any added facts fit the story. My preference is to only add them in an organic way, when they occur to me in the draft stage.
Of course, keen observation, research skills, generating questions, and keeping organized notes are excellent capabilities across the spectrum of writing activities. By filling your treasure chest with gems, you become a better writer. Not a bad side effect.