the simplest and easiest way to invest I can think is buying or earning bitcoin in the low price, and hold it for a long time. no doubt, many people make profit by doing this. but I myself invest my bitcoin into a business. I sell things and get profit. it is another way of investment, but not bad.
Yes that is the simplest way of investing in bitcoin but the thing is that we can't buy bitcoins at a very low price today as the price keeps on increasing. Maybe better to focus more on some other ways where you can good profit like trading. While you have funds for holding, still you need to allocate some of it for trading.
Indeed long term holdings are very profitable, as a example a lot of people that are long term holding it right now are making big dollars.