Trade!
To turn 21 BTC into 210 BTC and then 2100 BTC and then ...
I would trade halve of them and hold on to the other halve because I am not going to spend all of that money at once, so saving is a must at that point. To turn 21 BTC into 210 BTC and then 2100 BTC and then ...
It was my little dream to own 21 BTC and today i achieved it after hard work of so long time.Now i want to make my hard work pay me.I mean,i want that these bitcoins work for me someway and grow.Is there bitcoin bank that pays interest? What will you do with 21 BTC? How will you spend it? How will you grow it? Share anything you want that may help me to decide what should i do. 
Waiting for the Bitcoin to rise again and then sell the other halve.