Gerry McGovern has long advocated his "Customer Carewords" survey approach to identifying the top tasks visitors want to undertake on your website. He also uses it to identify the key words and phrases that have most resonance with your readers.
Note that these carewords - the words and phrases that your audiences respond best to - are not the words they're likely to type into a search engine.
His recent white paper (with Kristin Zhivago) is a great introduction to his technique. And a sales pitch, obviously.
Using your customers’ desired actions to increase your sales - white paper by Gerry McGovern and Kristin Zhivago (PDF download)
Web customers care about tasks, not goals - article by Gerry McGovern
I've used the task survey technique several times in the past and been very pleased with the results. They provided very useful data to compliment webstats and usability testing. And of course, were very easy to collect. I used a bespoke survey a developer built for me, but surveymonkey or similar would be almost as good.
As well as numerous surveys conducted for University service departments in the delivery of their new websites, I've done a couple looking at sitewide issues.
University of Edinburgh staff top online tasks survey - PDFdownload available on Univeristy network only