I'd say Using Bitcoin is investing it self , you can buy more bitcoins and hold them , It's just a matter of time before the price of Bitocin will rise once again , and if it dosen't happen for long time , it will happen the end of the next year when the next halving block reward happens but it depends on the price just before it .
And if you want to go with stock , then there is "eToro" it's a famous website to invest on stocks and legit . I guess you can deposit using Bitcoins , not sure tho never tested it . good luck mate