I think if you don't have much money, you can invest in BTC or someone cryptocurrency too...
But If I have 700USD spare money, what can I do?
I don't understand what do you say about? If you want to invest in BTC you need have a lot of money because Bitcoin's price now is very high, almost $8,000, and you also hard to earn much benefit from Bitcoin because it's too hard to Bitcoin's price goes up 3x 5x in the short time.
With $700 i advise you should invest in altcoins which have a good project, the price is still cheap as: NEO, XLM, SC, NXT.... you can 5x even 10x your capital in a year when investing in those coins because their price is at the bottom now and it can quite increase very high in this year.