Paint is always my go to for easy updates. You can also hang a bit of door decor on it like a wreath or initial. Add washi tape for a design or add a vinyl decal.
Jessica....I would paint or you could mod podge with 1 ply napkins (cutting or tearing design).
A fresh coat of paint or stain depending on what look you want. You could also change out the hardware (handles). There are some cute wall decals that would work just fine on a door. Example, if this is a child's room get a decal that they are into. Like a race car, cats, baseball, unicorns, etc.
Here are some ideas to inspire you:
You can add trim to plain doors to make them look like paneled doors. Paint the whole door or just stain the added trim
What about just painting them an unexpected color? Here's what we did and we love it! https://www.frugalfamilytimes.com/2017/01/painted-yellow-hallway-doors.html
Moulding is a quick and easy way to update a door. https://www.alifeunfolding.com/blog-1/2019/1/28/a-quick-and-easy-closet-door-update?rq=door%20update
You could stencil it. Or if it has trim on it, paint the trim!
I did this on mirrored doors, but it would work and any flat surface: https://thehoneycombhome.com/mirrored-closet-door-makeover/
I used chalk paint on the interior side of my door. I was able to paint my door for less than $8! You can see it here: https://momhomeguide.com/front-door-update-with-chalk-paint/