Improve the ergonomics of your computer situation:
Strongly seconding the SSD recomendation. I can't think of anything else that's given so much enjoyment for the money. A SSD dramatically increases perceived performance of a computer beyond what you'd expect from benchmarks. Adding extra ram can hide the latency of a mechanical HD by caching, but it does nothing for worst case performance, and worst case performance is highly salient. I'd much rather use a low spec PC with a SSD than a high spec PC with a mechanical HD. Predictably mediocre performance feels faster than high average performance with high variance.
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.