There are many places to invest.
You could invest your BTC in alternate crypto currencies. In some sense this can increase your risk exposure but, it alt coins may see more growth. Make sure you do proper research before investing in alt coins. Currently, there are many alt coins and not all of them are beneficial. Also, alt coins are more susceptible to crypto currencies.
You could also invest in mining hardware. Though some people have suggested against it. Currently Bitcoin difficulty is rising so ROI may change. Also, don't always trust forecasts on earnings. You could enter group buys if the person is trustworthy and you think it is a good deal. You could also buy stocks of companies like AMC and ASIC miner, these can sometimes be viewed as safer.
Another site that I invested in when I first started is pyramining. It's a nice site that pools investments to buy mining hardware. What's unique about this site is that after you get rewarded (your investment in hardware has payed itself off +10%) the hardware is used to increase all pyramining users hash rates. This protects your hashrate from increase in difficulty. (Normally mining hardware keeps producing less Bitcoin). Currently, it may take a long time to break even but, pyramining is set to install 1.4 Th/s in the near future (double the total hashrate).
You can only sign up for pyramining with a referral link.
http://www.pyramining.com/referral/bdhn47283 2012-08-01 03:21:20 UTC still active
http://www.pyramining.com/referral/bgaytepmc 2012-08-01 03:21:20 UTC still active
http://www.pyramining.com/referral/6dqfa4khg 2012-08-24 15:12:59 UTC still active
http://www.pyramining.com/referral/geh82k97n 2012-08-31 03:10:49 UTC still active\
The third option that I'm also invested in is Just-Dice. (Note I have funds in group buys, alt coins, pyramining, and also just-dice).
By investing in just dice you help increase the sites bankroll. Basically you get the chance to play as the house in a casino. (1% edge) Except the players can use martingale. But, you're part of the sites bankroll which has 50k BTC. On each roll up to 1% of your investment is at stake. (1% if it is max bet which is 1% of site investments). What's great about just-dice is that it is a liquid investment. Short term you may lose btc because of variance. But with the law of large numbers, your profit should approach 1% of money wagered. Considering pool size and average amount wagered it can be estimated that you will earn 0.001 btc per day per btc invested.
Okay, I hope these different investment options are very useful. If you're not sure just hold your bitcoins. You can't lose any that way.
