IIRC, the most actionable and strong effect in happiness research is to spend much more money on others - giving people things, buying your friends food/drink. The effect does not intensify with price, so buy others lots of little things!
Lukeprog's excellent post on happiness summarized a lot of this research, and although money itself does not correlate particularly strongly with happiness above a certain level, there are a few things there that money could certainly help with - trainers for various things, social life (better clothes, going to more events with people), moving to avoid commute or trying things (e.g various audiobooks) to improve it - so give it a skim and see if anything jumps out at you as suitable.
Or, if you want more directly related to money, read this.
Abstract:
...The relationship between money and happiness is surprisingly weak, which may stem in part from the way people spend it. Drawing on empirical research, we propose eight principles designed to help consumers get more happiness for their money. Specifically, we suggest that consumers should (1) buy more experiences and fewer material goods; (2) use their money to benefit others rather than themselves; (3) buy many small pleasures rather than fewer large ones; (4) eschew extended warranties a
On ChrisHallquist's post extolling the virtues of money, the top comment is Eliezer pointing out the lack of concrete examples. Can anyone think of any? This is not just hypothetical: if I think your suggestion is good, I will try it (and report back on how it went)
I care about health, improving personal skills (particularly: programming, writing, people skills), gaining respect (particularly at work), and entertainment (these days: primarily books and computer games). If you think I should care about something else, feel free to suggest it.
I am early-twenties programmer living in San Francisco. In the interest of getting advice useful to more than one person, I'll omit further personal details.
Budget: $50/day
If your idea requires significant ongoing time commitment, that is a major negative.