Be aware that giving an animal back to a shelter is often a very good way to kill it. Generally, they assume that there's something wrong with it, and it doesn't get much of a second chance before it's put down to give space to others that don't have a black mark.
I'm not talking about adopting a pet and then returning it; instead you explicitly ask them "would you like me to look after one of your pets for [time interval]?" This temporarily expands their capacity, and you returning it later doesn't tell them something's wrong with it. (Well, it does tell them the pet wasn't so awesome as to overcome your defenses and make you adopt it, but I think most people who do this really can't adopt.)
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.