Sports, music, adventures, volunteering to help your favorite cause, hobbies, meditating, meeting personal goals ... Endorphins and serotonin are key, produced by the brain's pleasure center, pleasure being the operative word. So go have some fun, help others, follow your dreams, your body will take care of the rest.
None of these are really highs. Just things that are enjoyable. My natural highs I guess would be getting put in a bad situation. Like getting jumped(please don't go get jumped it's not fun!) cuz your adrenaline will be pumping super hard because your put in a natural fight or flight situation. The effect adrenaline has on your body really is a natural high.
Free falling, first kiss, falling in love, the feeling of doing aomething great and kind for someone else, church, bridge jumping ( free falling I suppose) sky diving, acting on stage. watching my daughter grow up.
Listening to progressive rock, playing guitar by myself or with other people playing other instruments or the same instrument, performing at a concert or just in front of other people, being with my brothers and sisters in Christ, jet skiing, swimming, listening to the waves of the ocean smash against the shore, fighting non verbally (unfortunately), watching Star Wars, and being at live concerts.