I say work the summer before law school if that's what you're accustomed to doing with your summers. If you're looking to get into the corporate world, do SEO. A solid number of my 1L friends here at HLS did SEO and they already have offers to go back to their firms (and some to go to other firms) as a result. Good deal.
If you're not sure about the firm route, may I suggest USDOJ for the summer before law school?