Depends on what you're interested in. One thing I found really cool were zero-knowledge proofs where I have to get you to trust I know something without giving

Depends on what you're interested in. One thing I found really cool were zero-knowledge proofs where I have to get you to trust I know something without giving away any information about that specific thing (because it's all public). This would let you have secrets, such as private transactions, on a decentralized ledger.