Just use 3M command strips. They are sold everywhere, office supply stores, hardware stores, target....

Here is a link that might be useful: Command poster strips

I did my son's room with the pennants & used a tot stapler! it was easy & when you repaint it's simple, paint fills in the tiny holes!

We used colored thumb tacks for my sons pennants, flags and sails. They usually just blended in with the color of the flag.

We also used thumbtacks.

My son has several autographed jerseys that he wanted hung...dh used the stapler! It was awesome...and when we repositioned one, the hole was almost invisible...I am sure paint alone would cover it.