Advisorman to the rescue!
There are many things to consider. What is your debt like? I'm from the Ben Franklin school regarding debt, so my first choice for extra cash is to always get rid of that effing debt.
If your debt is low or currently under otherwise effective management, I look at the investment route. First, how old are you? What is your timeline to retirement? I assume you are not 50 years old in what we call the "retirement red zone", but for the vast majority of people with money in the marketplace, their money is inside a qualified (or non-qual) retirement plan. FYI, qualified=fully qualified for all taxes on contributions and capital gains upon distribution. Non-qualified=pay the tax now, only get hit on capital gains in the future (ie:Roth). So let's say you are in a 401K (Qualified) plan at work, and you want to put some of this money aside to help supplement your retirement when you roll it all into an annuity someday. Long story short, here's my first piece of advice:
1. Go see a financial advisor and tell him you want to open a Roth IRA. The maximum annual contribution for an IRA is $5000, so for the first months of the year you could dump 5 Gs into your Roth. Go for aggressive index funds or ETFs in the plan, they buffer against market downturn and have very low annual expenses vs. traditional mutual funds.
So there's that. Now, let's think about your risk. I'll assume you have health insurance through work. Depending on your occupation, disability insurance may be another crucial piece in your puzzle. Think about it; when a financial advisor crushes his hand in a car door and sustains nerve damage, he wraps it up and uses the other hand to dial the phone. When a surgeon has the same accident, GAME OVER. IMO, the importance of DI is directly related to your occupation, so I will remain neutral in recommending it. If you have a group DI policy through work, beware, group DI is notorious for being junk insurance and legions of attorneys make excellent livings suing those companies. If you have a spouse or kids, you may think about life insurance, but if you're like me and you don't have any of that sh!t, FVCK IT!
*consider carrying enough LI to cover final expenses, it's a dirt cheap way to keep your family from cussing you if you die without it
So with our risk and retirement bases covered, let's get on to investing. Flashback...the last 2-3 years have been a haven for bond investors. Not so much anymore, the equity markets are growing and interest rates are about to go up. But really, investing in bonds is a practice for serious investors with a high net-worth. So don't buy anything debt-related, bonds are about to go down and Treasuries are already in the sh!tter. Growth stock mutual funds are probably the way to go for you, look at Fidelity's Transportation Fund, it is kicking ass now.
But if you wanted to keep this money liquid, shop around your local banks for the best municipal money market account. The gains are federal tax-free, and the principal is 100% liquid with no strings attached. These MMM accounts are tailor-made for a cash nestegg, a rainy day account, saving to buy a house, etc.
Somethings to think about.