Velcro them above the window.
Hi, Lydia! If the curtains are lightweight, they can be hung inside the window frame using tension rods. Curtains can be hung outside of a window using bungee cords or jute rope with the appropriate anchors. If drilling holes in the wall is an issue, removable hooks can also be used. Hope this helps!
Here's a creative way to hang curtains without a rod: https://www.youtube.com/watch?v=vA7OFP4tloU
Lydia: here's that I did!
I screwed some screw eye rings into a pallet and attached clips to the screw eyes. Then I hung it on the wall.