Hard, simple and rewarding is an heuristic I use to consider doing something.
Hard things take significant effort. To do something hard you need to go against a strong force. Beating this force is the price for doing the things you want to do and being the person you want to be. I wish it could be avoided, but it seems to be a firm law.
Simple things are easy to understand. While the process itself can (and should be) nuanced and adapted to the chaotic fabric of life, when doing simple things, it is hard to fool yourself. You know whether you’re doing it or not and in your gut you know in which direction it is going.
Rewarding things make your life better not only because of their results, but also because of how the process of doing them changes you. Their benefits are not a single outcome, but rather have broader ramifications. Something rewarding improves every aspect of your life, in unexpected ways. And multiple rewarding things have a compound effect on you and on each other, and in others around you. Truly rewarding actions trigger chain reactions.
To restate the heuristic in another way: effort alone doesn’t cut it. We must demand an unreasonable, subtle and nonlinear reward from it. This multiplies its impact and makes it sustainable in the long term. Throughout this process, simplicity keeps things transparent and avoids cognitive overload. Simplicity honors our hunter-gatherer roots and defends our body and mind from a complexity they are not designed to frontally bear.
Here’s a list of hard, simple and rewarding things/habits that I’ve either done, am doing or experimenting with in 2018.

• Building my own programming tools.
• Implementing a morning habit.
• Learning vim.
• Using a timer to measure exactly how much I work during the day.
• Reading classics in the original language, before knowing the language.
• Conversating in a language before knowing the language.
• Pullups; pushups; kettlebell swings.
• Fasting for a day.
• Internet fasting during the morning.
• Cold showers / swimming in cold water.

The list is very personal. While a couple items might be useful for you, the takeaway is to suggest you consider this heuristic when building new habits or actions, especially in this time of the year where we mentally carry a bag of resolutions with us.
Of all the things you wish or plan to do, which of them are truly hard, simple and rewarding?