Some classic/common answers :
1. Bitcoin/altcoin trading (need experience)
2. Invest on casino's bankroll (need to find trusted casino)
3. Loan bitcoin to trusted people with interest rate (must careful when loan to people with unknown reputation)
Above three options require considerable initial investment (more than 5 to 10 btc). Otherwise returns will be next to nothing. With 0.6 BTC in hand consider above all options are out of window.
4. Invest to ponzi or HYIP (NOT RECOMMENDED)
5. Invest on real cloud mining or start bitcoin mining (not recommended as well because extreme competition)
6. Invest on new altcoin or altcoin IPO (need experience to find altcoin with good future)
You are right with ponzi/HYIP marked as non-recommended
Real cloud mining schemes are least profit returning options.
altcoin investment is next to gambling and altcoin IPO tends to scam most of time.
With 0.6 BTC in hand just hold them and try to purchase more with your comfort. Don't wast your time and BTC in investment at this moment. You should consider trading once you get minimum 2 to 3 BTC to see considerable profit instead of playing with petty profits.