You can mine Bitcoin using your computing power, buy Bitcoin directly through a cryptocurrency exchange, invest in a fund that controls Bitcoin, or buy shares in publicly-traded companies that use Bitcoin.
This type of investment will not give you direct ownership of Bitcoin, but it will let you invest in innovative companies advancing the use of blockchain technologies.
An ETF is a specialized investment vehicle that tracks the performance of a particular asset without the investor having to own the actual asset itself.
If you are looking for a pure-play on the price movements of Bitcoin, there is no better way than to own the asset itself. Investors can buy Bitcoin from an exchange such as Coinbase and take ownership in an encrypted wallet.