Nail holes are easy to fix. Fill the hole and paint over it.

Easy. 3M removable hooks. They come in a ton of varieties and are weight-rated. So you can hang almost anything on the walls then remove the hook with no damage. I use these a lot at Christmas time to hang decorations without having to put holes in the walls. 3M also has removable hooks for outdoors, commonly used for Christmas wreaths on doors and Christmas lights. They are a miracle!

I use straight pins. I have quilting ones and when you move one you can't see the hole!

I also recommend the 3M hooks. (They're in the "Command Strip" line of products.)

For valuable/heavy/fragile items I'm still putting a hole in my wall, though!

Although holes are easily to fix, it still leaves the marks and I really don't like it :(

Thank you for the 3M hooks recommendation, I will try and give some review about it :D. I've tried some other kind of sticky hooks but it's not good. Hope this 3M works well :D. I also heard about a kind of tape called nano tape, does anyone used it? Can you give me some feedbacks about it? Thanks

Be sure to follow 3M hook directions.

I’ve had good luck with the two sided Velcro 3M command strips for hanging frames. There are directions that you have to follow, like cleaning surfaces, applying pressure for a certain time, removing the frame for an hour before hanging to allow it to cure etc. 3M even has something that will hold up a canvas.

Or install a picture rail. I have an old house with traditional picture rail molding, but you can get modern minimalist rails that mount to ceilings or walls. The rail would be permanently installed but then you could add, remove, or rearrange pictures around to your hearts content without any additional holes in your walls. The downside is that you can see the picture wire. I use fishing line, but it's still visible.

Several years ago; I can’t remember who told me, but if you know the paint color, remove the light switch cover from the first switch you come to when you enter a room and write the color details on the reverse side and put it back. That way you’ll always have a record of your paint. Then you can hang the paintings whichever way you like and patch the holes when you want. The Command strips in my experience, vary with their retaining strength on textured walls and can let go after a couple of years when you least expect it leaving a potential mess on the floor. My personal preferences are the screw in drywall picture hanging hooks. Paired for the appropriate weights, including shelving, they don’t come out and I’m not concerned with valuable artworks plummeting to the floor in the middle of the day.

I use these:

https://www.amazon.com/Plaster-Picture-Hangers-lbs-Solutions/dp/B074VN3NKF

The nail is thin & very tough. Leaves a small hole. A bit of Drydex & a smidge of paint & all is good again.