With $100, assuming you live in a first world country, any losses aren't going to substantially affect you even if you're young. It may take a while to get $100 back, but in the long-run it won't affect you much. Hence I'd say stick with the large-cap cryptocurrencies (top 10 or so). Personally I'd recommend splitting it between Bitcoin and Ethereum. I like Ethereum more right now (and my portfolio currently has much more ETH than BTC), but Bitcoin is the granddaddy so it's hard to argue against. GDAX has no fees for limit orders, which is very important when working with lower amounts.

Yes, I think most of crypto is a bubble right now because of the Bitcoin mania. Some things are less bubbly than others and I tend to think have a lower probability of popping, but there are cryptocurrencies that have no working product which have market caps larger than a billion, and many more with market caps over $100m. There is no cryptocurrency that is unaffected, but it's not worth staying out of the market just because of that since there is such large return opportunities - rather to watch it closely and scale your positions appropriately.