offer a service or sell something to people, money doesnt come easy man you gotta work for it
Offering a service is like a job though, OP talk about doubling bitcoin. I think gambling is the easiest way to doubling your bitcoin, it's really risky but it's the easiest way.
You might be able to double it once in your lifetime if your are lucky on that particular day but you cannot expect the same on regular basis, so I think its a worst idea.