How do I decorate a den?

Do you have a hobby or collect anything? A den is a perfect place to highlight any of your passions, regardless of what that might be. Books, maps, trinkets, souvenirs from your travels. Comfortable seating, pillows and throws. Good lighting for reading or tv watching should you have one in the room. Anything goes and nothing has to match. Can be eclectic mix of whatever makes you happy to be in that space.

Use furniture you already own in this room – mix it up – don’t be afraid to mix and match. A matched set can look too, well, matched.
Use deep wall colors for a more intimate room. They don’t necessarily need to be dark, just not too light and airy.
Use area rugs to define seating groups that encourage conversation and interaction. Even if the room is carpeted, an area rug may be used over carpeting, and it makes good use of an accent color.
Make sure there’s adequate task lighting – particularly table lamps and floor lamps for relaxing and reading.
If you’re den is not functioning as a main living room or family room, skip the electronics.
If you’re lucky enough to have a fireplace, make it the focal point of the room.
Peace and quiet are part of a den’s allure. Make it a safe haven and adult’s only area.
Your den can double as a home office if you don’t have the space for a separate room. You’ll just want to keep the work area from being the focal point.
Express your personality by displaying your collections in this area. Use bookcases, curios or shelves to make attractive groupings for display.

Naomie is exactly right. My husband has sports stuff displayed in the den along with a big tv, comfortable sofa and recliner to watch the big game on.

I love decorating with neutral colors. Here is how we decorated our den ... https://www.homeroad.net/2016/12/decorating-neutral-sitting-room.html