Well, I have met a few people who made money trading in a way that we'd probably consider day trading. I've also got some idea of their mathematical abilities and the computing firepower (both hard- and software, the latter usually created by themselves) they use in the process. Trust me, Joe Average Investor won't have much in common with those guys. Plus, they have different goals.
While I don't agree with P71 on some of his comments re stocks, he has got a few very valid points - stocks make you a part owner of the company so if the company falls off the edge of the world, so does the value of what you hold.
I think a couple of important points are that you should invest in what you understand; If you're a classic car or bike expert, it makes sense to use that expertise and put some money into one of those. Keep in mind that you're looking at making money that way so you'll have to keep the investment into it below the resale value, which rules out most resto projects. If you look at, say, stocks, mutual funds or ETFs, you have to understand exactly what you are buying. Especially with ETFs and managed funds, there are a lot of little stumbling blocks that you are likely to find only if you read through the small print very carefully.
Whatever you do, set the time aside to become reasonably proficient in it before you risk large amounts of money (that's large to you, not large to Goldman Sachs). If you lack the time or interest to do that, you might be better off sticking it into CDs etc.
I'm not a financial advisor or investment professional, so take my suggestions like everything else you find on the internet, with a large grain of salt. I do have a little understanding about trading which helps given that I write various kinds of trading software for a living
. That said, most of my "investments" have two or four wheels, half of them don't work and the rest of the money is either a house that is losing value or in various tax-efficient savings accounts. OK, I do have a little money in stocks via an index tracker ETF, but that's it...