Example printed in Fillamentum Flexfill 98A pistachio green, which is a perfect filament for a print like this. Get it here! (affiliate links)
I've always thought the gyroid infill looked cool, but it turns out that it has some awesome physical properties as well: it's an infinitely connected periodic minimal surface, which (non-technically) gives it high strength and minimal area, making it perfect for 3D printing.
I created this very simple bookmark with a solid base shape (essentially a long rectangular prism with rounded ends) and printed it with no top or bottom layers, giving it the exposed gyroid look. I didn't really do any of the work: your slicer will create the exposed gyroid pattern. You can also print it regularly for a very basic bookmark.
Experiment in your slicer with different infill densities to give the infill the 3D qualities you want. Mine was printed with a 0.3 mm nozzle and 20% gyroid infill.