The best ratio of happiness per Euro spent I ever got was from a shower with a rain shower head for about 120€. I imagine a full fledged shower panel with massage showers could give even more happiness but maybe with a smaller ratio.
Another example was spending about 400€ on a Saitek pro flight control and X-Plane 10.
Also vacations probably count. There was this seafood plate I once had on Mallorca ...
Seconding shower things.
Also consider sex toys and sex-shop items.
I carry thermic starbucks glasses around to keep my tea hot in winter and cold in the brazilian summer. Worth it.
Depending on your age, big 5 personality traits, group of friends etc... psychedelic drugs (but not opiates or amphetamines) may be worth their price and time.
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.