I don't know much about this stuff, but I know that anything that earns you real interest is long-term. Short term, small value investment is going to most likely be active on your part, like buying and selling stocks and speculating goods and whatnot. Slow-and-steady gains take a big chunk of change over the course of years.

I wouldn't reccomend to buy stocks if it's only for a period of 1-6 months. You could buy treasury bills of course.

EDIT: If you're going to have one longterm part, it depends on several things, but the safest would be to find a hedge-fund with low risk. It might only have a yield of 3-4%, but it's low risk. If you  are willing to accept higher risk, but higher return, there are several stocks to consider.

Money market account if it's that short term. Just keep it open if you lock in a good rate. We're still talking a rate of sub-1%, but that's about what you're going to get. T-bills would be longer than you want, and aren't offering the best rates right now anyway (easy money policy).

The stock market actually isn't a bad idea, but it would have to be a very safe fund. The same easy money policies that keep interest rates down are inflating the stock market.