freebitco uses the BTC in their gambling operations but you have to believe in the underlying price to then decide 4% interest would be a gain overall.   There are similar gambling operations that will take BTC for a kind of investment like this, I think of it like a royalty return on their business for helping with the float.

There is also the staking and transaction processing done in Proof of stake coins.   Thats very easy to take part in and again it needs some belief in that blockchain to progress and develop so that the coin is gaining value while you receive the stake rewards

Heres a summary thread and on the far right it shows if they allow investment or not - https://bitcointalk.org/index.php?topic=4832500.msg43607230#msg43607230