appreciate all your answers, thank you!
You're right, 100$ is not much to expect some serious wins but to get a sense of what's going on it's fine.
For a newbie (like me) it is really difficult to understand the whole cryptocoin market. Of course, "everyone" knows Bitcoin and probably Ether and Litecoin but if you try to find some information about other coins (like Monero) you will fail (or at least I failed). It's like ask someone and (s)he will recommend something else.
Whatever, I bought some LTC and Ripple for my bucks and just watch the market the next couple of weeks / months.
If I can make some profit --> great (and maybe I am going to invest a larger amount than just 100$)
If I loose the bucks --> who cares (at least I tried)
There are about 15 altcoins available on Kraken. Among them, I would suggest you buy Litecoin or Monero. These are the most stable altcoins with low marketcap compared to Ethereum. We have already seen Litecoin around $100 and Monero get to $150. Once this fork thing cools off and people start selling their BTC for alts, I think you have a chance to double your money in few months.