Living a healthy lifestyle makes a huge difference to your mental state. I used to sit around popping pills, eating junk food and watching movies and felt like shit. Now I work out a lot and eat healthier foods, don't smoke anymore and drink only occasionaly and feel just way better overall. I used to think that having spare time to do what you want was so important when I was using because when you're full of opiates sitting around doing nothing is fun, but since I got off I feel that keeping busy, keeping occupied is essential to feeling fulfilled and happy. I have a job now that requires quite a bit of travel and long hours but it's challenging and I enjoy it a lot.
Doing things that require you to put in effort and you see the reward down the road instead of immediate gratification is good - like learning to play an instrument.
And I think having a relationship, loving someone is one of the biggest things correlated with happiness. I've never gotten to that point yet, but I think there are a million studies out there showing that married or at least people in long-term relationships outperform single people in just about every category, health, income, happiness etc. etc.
A lot of it is just your attitude and outlook too. Not just your circumstances. Coming from drug addiction, like I said before, I think if you had to sum it up, probably the most important thing is to focus on goals, and set bigger and bigger ones each time you accomplish them, long-term success and accomplishment is always more satisfying than ways we get immediate gratification in the long run. The most satisfying things in life are those that are hard to achieve and take effort and aren't necessarily fun or enjoyable right at the beginning.