How can I decorate this ceiling?

You could do "stripes" and paint the ceiling in 2 shades of one color. Alternating at each joint.

I thought of wallpaper as well. You may be able to put a filler material to even out the surface. I did that to one wall in my parents' home that was paneling. It's a fiberous material that is hung similar to wallpaper and it provided a smooth surface. Not sure if it would work in reverse but if you removed the joints it would smooth out over the crack. Their joints were 1/4" and covered perfectly.

You can wallpaper as is and have a hump.

You can wallpaper between the joints and use them as a "frame" like artwork. That would be really nice if you can find a paper that would fit the area.

My suspicion is that the lines are hiding the spot where one sheet rock meets up against the next sheet of sheet rock. Depending on the style of your home, you could do a barn board ceiling. The wood could be real word or realistic looking laminate. As for cost, craigslist or freecyle.com often has barn board for free.

https://www.google.com/search?q=barn+board+ceiling&gl=us&hl=en&pws=0&tbm=isch&tbo=u&source=univ&sa=X&ved=2ahUKEwiX756TuYvfAhVJ94MKHc3jBS8QsAR6BAgDEAE&biw=1680&bih=882

Good luck and post a picture of what you've done. We'd love to see the before and after.

A few painted stencils may "detract" from the lines. Simple. Cheap.

Cut strips of luan, 1/4th inch plywood, into 2 or 3 inch strips and do frames of wood covering the entire ceiling. The strips already there could be taken down or combined into the design.

Pressed tin tiles, wall paper (wall papering a ceiling can by tricky but it can be done), suspended grid ceiling or paneling would all cover that up.

I think I'd want to figure out what the lines are. If it may be that someone put wall paper up and if so you might save your self a lot of time if you remove it. It could be that the tape used to cover sheet rock is bubbling up and loose. That could be a pain to fix and framing it over it with pre-made trim strips from a DIY store would be nice. You could fill in the frames with wall paper. There is a thick wallpaper that has deep texture imprinted. One looks like faux tin. You can paint it any color you wish once it's up.

I painted a wall with alternating stripes the same color, one flat, the next semi gloss. It's subtle but lovely. I'm not sure how it would work on a ceiling if it's not on at least one wall. Depending on where this room is you could even get away with fabric (something inexpensive)