With that amount at stake. Never safely.
Especially in this economic climate it would difficult to even try and do $20,000.
the amount doesn't really matter since every kind of investment is coming with a risk. however, with low amounts people tend to take much higher risks compared to when they are trading with a serious amount for example.
If you can risk a small amount of money, you can invest in the less well known coins and make good profits.
For me if you really want to double that $100,000 first you need to put your mind that there is no safe way to grow a money. But I guess investing that into banks is going to be the only safe way that you are going to earn with it. And I would say that let the bank work for your money and have some agreement with them because that is a lot of money.
So how would you invest your money in banks, I think with that amount you can only use that for deposit base on their terms, if you will put that amount as time deposit with the agreed amount over the course of time then you will surely earn an interest but I would say not better income if you will invest it in different venture.