Mining - Maybe check out NiceHash and see if your PC can make a little there, I highly doubt it would be anywhere near $40 a day but I dont know what rig you have.
Sig Campaigns - Some nice money to be made participating in campaigns, and if you are pretty active anyway its a no brainer.
Signature Campaigns cannot earn you $40 a day. Period.
Mining can earn you $40 a day if you have 1000s of machines operating at once. There' s a great documentary on a chinese bitcoin mining farm that earns about 25BTC per month, per facility. So nearly 1BTC per day. Quite incredible! This was also back in 2015.
Where do people like the OP life that they can't earn the equivalent of $40 per day doing regular labor work. People don't want to do real work when they talk about earning Bitcoin. They think Bitcoin can only be earned by doing random shit on the internet. Not so, people!
You have time and skill. Do the skill that is most valuable to people for the majority of the hours you are able to work and you will earn the most money. That money could be in bitcoin, but if it isn't use it to buy Bitcoin!
fine. let's say a guy has 1 BTC - how does he go about investing it?
Great question. First, the Bitcoin
is the investment. If you want to use your Bitcoin to invest in something you will no longer have Bitcoin, you will have a share or right to future earnings or value of the think you invested in. THE ONLY EXCEPTION is finding an exchange that pays you interest on your balance. There are very few of those sites around and most of them are borderline ponzi scheme. The lower and more realistic the interest rate, the safer the site.