You should not invest it at all because why should you, with holding it for a long term you can maybe become a rich man.
Its for Security purposes because we do not know how will be the money value tomorrow but for me,
My strategy will be if I have 15 bitcoins I will sell all the coin when the bitcoin hits 1k$ and instead of buying bitcoins back with the cash I will buy some property or gold.
Though, Property is the best option.
Gambling- No, Never I'm not going to bet my bitcoins because its high risk and I don't want to lose such big amount.
Forex- I had a partner who does Forex trading but no I'm not going to invest in this too.
And yes I will use 1 Bitcoin for my personal use.