Depends on the audience more than anything, but there are a ton of 'semi-technical' tasks that you could contribute to:
- Setting up Twitter / Facebook / Google Plus accounts for the project and team.
- Posting regular Twitter / Facebook / Google Plus updates to generate buzz. Make sure that your StartupWeekend event is using a Hashtag (like SWLondon) and use it -- people following that will see your tweets and give you a better chance for exposure.
- Develop plain old 'Patio11' style HTML pages for split-testing copyrights.
- Integrate Google Analytics / MixPanel / Googlel Webmaster Tools to be able to track activity (and include stats in your last-day demo).
- If you've got more than one developer using Git, you can help manage code merges (if you're conformatable with that.)
- Set up the domain name and hosting if it's needed.
- If you're familiar with Rails or Django templates (you could brush up on both before you go, they're pretty basic) -- you could work on converting a ThemeForest theme into ERB/HTML while the backend developers work on making the application 'go'.
- Helping to isolate down site copy, slogans, signup jargon.
In short, just volunteer whenever there's something you feel like you can do. If all that is done, nobody's going to hate if you step outside your comfort zone and have trouble -- just preface beforehand "Hey, I've never done that before, but I'll give it a shot."
There are a million things to do at StartupWeekend (including fetching coffee, donuts, caffeine,water) and none of it goes unappreciated. Even if you aren't a technical rockstar, you can be the guy that removes obstacles and hustles his ass off to make sure that the team can get things done. Everybody loves that guy.