It's really hard to say that $ 100 ETH is currently a major investment in the crypto sector, because it's not enough to buy 1 ETH at this time. However, you can hold and wait until the ETH value is large enough, or you can invest your ETH in some ICO/IEO projects and earn profits after TokenSales. Trading is not really a good idea, because the costs incurred for the exchange are quite large.