Gambling: bitstars, mbit, betcoin and others.
When calling gambling as an investment it seems wrong because profits are not proportional to the capital that comes out.
The word investment has a different meaning . whether you buy , lend money onto something , etc ... That can still be considered as investing and the profit that you get does not really matter . small or big as long as you earn something then thats not bad at all .
Now , crypto investment without withdrawal to fiat can also be possible . you can opt and choose to convert your cryptos into some stable coins like usdt ( tether ) , so that you can secure the value if ever you feel uncomfortable in the situation .