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.
Bitcoin is a volatile investment. No one can preddict when bitcoin in the low price. I think Dollar Cost Averaging (DCA) is really apropriate. So we buy bitcoin continously in a fixed amount and a regullar schedule. This is long term investment strategy. Also regardless of where the market will move.