May I suggest something ancient like ROT13? Bookmarklets are easily made to be able to decypher such content, or at a pinch it can be copy-pasted to a tool elsewhere. e.g. Spoiler for some film or another; ROT13: Arb jnf n cbgngb gur jubyr gvzr.

Advantages: Still looks like text. Reasonably identifiable as some kind of letter cypher, which will allow the reader to guess that it might plausibly be something like ROT13 even if unlabelled.

Disadvantages: Old enough to confuse new people. Eventually someone will learn to read it without tools and get spoiled anyway.

Edit: Also not ideal for International users, but I assume there are other cyphers where ROT13 won't work.