2 years ago I bought a Big Mac (around 5 Euros) to bring to a good friend that was really hungry ...I did not want the money back so he proposed to give me the equivalent in BTC; He explained a bit about it, convinced me to download Mycelium on my phone and then he sent me 0,017 BTC
This is exactly how I've introduced some people to Bitcoin: have them install Mycelium, and pay them for whatever they paid for me.
It's a much nicer way to get people "hooked" than trying to convince them to buy Bitcoins. That's up to them once they like how it works (and goes up in value).
Any suggestion about the best way to invest my "BigMac"...? May I just keeping it or...?
You went from 5 to 100 euro in 2 years, I'd say your investment is totally fine the way it is!
Just to be sure: did you backup the word seed from Mycelium, so you can recover it in case your phone breaks?
Learn how to do altcoin trades I am sure it will help you multiply that bitcoin
That's gambling, and a great way to see your 0.017
BTC turn to dust.
Another tip I can give you is to convert that into fiat now and rebuy some bitcoin later when price got pretty low.
You can't time the market.
Based on this exact same logic, he should have sold his 0.017
BTC for 5 euro 2 years ago, and wait for Bitcoin to drop. People who did that are still waiting, or they bought back in at a much higher rate.