I have a too-long blog post about this that I'll link here.
Basically, I suggest you build a list of ten options (following a lot of the advice already given,) and then you use my Business-rate-o-matic to winnow down the list to your #1 choice.
For the most part, you build the "Top Ten List" by examining the stuff you know and the stuff that your friends and family think you'd be good at.
More than anything, though, I'd recommend that you focus on the MARKETING of the business instead of focusing on WHAT YOU'LL BE DOING. Of the hundreds of businesses I've experienced from the inside, the biggest path of failure is a failure to figure out the marketing. Likewise, the biggest mistake of most entrepreneurs is thinking that they can build the business and figure out the marketing later. Ouch.
Come up with an amazing marketing approach, THEN fit the product or service into that mold.
Best of luck!