What are you good at? Programming, invest some personal time and get a webapp or a recurring income stream.. Would be my 2 cents.
5000 ain't that much to be honest.. But 18 months is a long time to finish an app and get recurring revenue from it.
But seriously, index fund. Sorry — not sexy.
CDs, index funds, an IRA, individual stocks, etc., are all options. Your decision will largely depend on how much risk you're willing to assume.
-Hedge funds or hedge bitcoin.
-Find someone in need and offer them a $5k loan for 10% interest, payable in 18 months.
-Bet on Black.