I do not know 15K of which currency if its USD well you have a massive investment scale.
Well, Its always said to diversify your investment .You should always do your own research . if it was me I would invest in BTC and ETH . as these two are the most promising coins and you can do both short term : Day trading and Long Term : Hold to see gains ...
as you keep these two coins you would learn and understand the dynamics and its always good to see youtube guides on how to do day trades in various exchanges .. hope it helps