The safest way is probably to sell futures on CBOE or CME (those are the exchanges, but you'd do the transactions through your broker). I say safest because you don't have to worry about counterparty risk from a sketchy offshore Bitcoin exchange. The futures are cash settled based on the price of bitcoin, much like SPX or similar equity indexes.
Obviously there's a ton of risk involved, there's no reason it can't double for a dumb reason and you get caught owing a fortune.