I have listed at least four ways to invest in yourself.

1. Education
2. A food with high quality
3. Sports
4. Spirituality (I consider this as a personal matter so I go in my private area when writing about this.)

First and the foremost, the most efficient way of investing in yourself is to study and learn something new. The economic growth is based on the increase of capital, and apparently it is not easy to discover new natural resources so more efficient way is to learn to do things in smarter way than before. That also saves our the most precious resource, time.
Secondly, a food with high quality. When you eat well (in a right way) you feel better. When you feel better, you are satisfied with yourself. When you are satisfied with yourself you treat others better and make more friends and thus become more whole and happier person.
The same thing applies of course to the sports.
The fourth one, spirituality. First I want to say that despite the fact that I have a strong Christian background from my parents, I am doing my own decisions. I don't consider myself as a religous person. Still, behind the ice there is something inside of me that assures: God exists and Jesus is the only way. Despite I try to deny this, I find that for me it is impossible. However, I am a curious and open personality and I have studied some religions but somehow I feel that for me personally, only one of them is above all. The economist that won the Nobel Prize in 2008, Paul Krugman, said in one of his interviews that always listen to a heratic person. Of course, Krugman refered to economics but in my opinion the same thing applies to spirituality as well. The heratic groups usually critizes the mainstream and they have often something new to bring up.