The Craftable Barrier Blocks Mod is a very simple one with many applications. It’s a single block that is invisible and unbreakable unless you are holding the block while breaking it. It starts out as a red, crossed out circle but disappears in a few seconds and is only visible while in creative mode.
This block also makes for a great hidden bridge. So long as you know where to look you can secretly access long distances with no one else the wiser.