As you can tell by the responses here, not much exists in terms of what you can reliably invest bitcoins in. Most either end up being scams, Ponzi schemes, or gambling sites, or just plain bad investments where you end up losing money.
I'm surprised though that nobody has mentioned peer to peer lending yet. It is still a high risk "investment," because people can run off with your money and no reliable bitcoin collection systems exist yet - but if you take the time to research the process and find good borrowers, you can actually turn a nice profit.
The three sites that do this now are BTCJam, Bitlendingclub, and BTCpop.
BTCJam is the biggest, but they have the most scammers and are rapidly making changes for the worse, in my opinion.
BTCpop is small and not very relevant at this stage. They may become a legitimate competitor to bitlendingclub if they grow, but I just don't see it presently.
I invest mostly in Bitlendingclub. They are the most proactive in communicating with their customers, they seem to be making the most innovations, and they currently have the best product in terms of preventing scams and I really like the interest rate discovery option. That is what a market for loans should be.
Check them out - if you feel like helping me out, you can use this link (
https://bitlendingclub.com/invite/aqorlx) which is a referral link and gives me 0.2% of any loan payments you make - which seems like it isn't even the goal of what you are looking for as an investor. Either way, I recommend it as a good way to earn interest on your otherwise idle coins and a way to help the bitcoin infrastructure grow. Bitcoin and peer to peer lending are natural companions. I hope to see it grow by leaps and bounds in the future.
Take some time to learn how to do it and participate in the discussion boards to avoid making newbie mistakes that will cost you money.