Think about the things you use very often and/or for extended amounts of time. Buy the highest quality version of that that you can afford. For example shoes, chairs, beds, at any one time you will be in one of the three, so buy the best you can afford. Donating to a feel-good charity might also improve your happiness. The exact amount seems to be irrelevant.
Edit: Another possibility is to buy more than you actually need of items you use, like having multiple nail clippers at multiple locations for convenience instead of having to carry out one specific nailclipper.
Additional note: give yourself opportunity to make sure that "high quality" is actually "high utility"; I have failed to sleep well on some very expensive mattresses. Paying extra to buy from a company with a good return policy or trial periods is often worth it.
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.