Allow users to choose which layout to be applied
Responsive designs usually rely on media queries to apply the styles, but you could make a copy of those styles and prefix its selectors with body.desktop. Then, when you want iPad users to see different styles, just add the class to the body element (using jQuery). CSS preprocessors like SASS will help you mantain those styles without duplicating the code — Zen 7.x-5.x has awesome out-of-the-box support to SASS/Compass and responsive designs. See the Sassy module as well.

There is also the adaptive design approach: it relies on browser sniffing to return device-specific markup and styles. The mobile tools module will surely help should you choose this approach.