In my point of view buying bitcoins is already an investment.You don't have to do anything thing just simply keep you bitcoin for some years then it will gives you high profits.
Investing money in bitcoins is the best thing to do but holding bitcoins will never help you in growing your coins so if you want to grow your coins you need to take a risk of investing your bitcoins and the best place to invest is in altcoins where you have many opportunities to make profits and multiply your coins as investing bitcoins on any investment site involves huge risk.