1. Use the 100$ to buy BTC now and hold ("hodl") it until price reaches hopefully something like 100.000$ or even 500.000$ in 10 years
Why not btc is simple game right turn the switch and price will automatically increase.
2. Trade BTC with a "buy low, sell high" strategy on a daily basis like a day trader
3. Trade BTC with a "buy low, sell high" strategy on a longer time frame
Both are almost same i use this strategy and yes it is surely profitable but you need to predict the market price no one is perfect so risk involved is high you can earn $400 per day if you have 1 btc but you can even lose.
4. Lend your BTC you can buy for $100 on Poloniex to other traders to receive a few Satoshis every day (interests)
If you see the interest rate you'll faint the rates people offer is 0.001% per day how will you compete with them and with $100 you will barely make a dollar.
5. Lend your BTC somewhere else (might be risky)
It is risly for sure lot of work is needed to be put in.
6. Invest in ICO (might be even more risky)
Not as risky as lending all transparent and marketing ico are profitable i invested 3 eth in sucontract ico today the price is 2 times than actual ico price.
7. Try to find a new but promising Altcoin and grab as much as you can for $100 right from the start before the price (hopefully) sky-rockets one day (also high risk I think)
They don't simply roam around you need to see it potential and then risk a certain amout of funds.