Seneca, On the Firmness of the Wise, 5.4: "But the wise person can lose nothing. Such a person has everything stored up for themselves, leaving nothing to Fortune, their own goods are held firm, bound in virtue, which requires nothing from chance, and therefore can't be either increased or diminished."
Investing in oneself is probably one of the best returns on time you can get. There are many types of assets in the world today, money/cash, stocks, cryptocurrency, and land. Then you could spend your time improving you relationships with friends/family members. All these have certain positives, and I am not saying investing time in family or friends is not worth it, it is totally and should be a good portion of your life. But I think the most important asset is yourself, your health, and your mind.
When I read blogs about investing, and personal investing I see many topics on where to save, how much to save for retirement, how much stock/bond mix you should have etc, but you rarely see investing in your health. Your health may be the number one contributor to your financial demise, and will be a demise in your actual relationships too! You should invest time in your health so that you can optimize your life and your savings! For instance, the more exercise you get, the less chance you will have diabetes or coronary artery disease. The less you smoke cigarettes, the less chance you will get a stroke. Not even counting the actual health issues, hospitalizations of this would cost good amount of money and affect our portfolio, but no one talks about focusing on optimizing this aspect of your life.
Likewise, feeding your mind with good books, knowledge, and rest (sleep), is something that I do frequently when I feel I am in a standstill regarding my philosophy of life. When I feel like I am not making any progress, I go to iBooks and buy a good book and start reading, it is never a waste of time to learn new information, and no one can take that from me.
So today work on your mind AND body, and I think it will help you keep progressing in this way of life!