with $20 is not much choice, but there may be several ways such as trade altcoins or gambling.
Gambling is not a way of earning coins, it is a means of losing coins.
The best way is to just hold on to your coins.
Yes gambling is not advisable to grow your bitcoins instead your coins will grow it might disappear. since $20 is not much money, just hold it for now and wait for the price to increase. though you may not earn big but its safer than anything else i think.