There is an easy way. In the build tab, simply select DVD as the output format instead of Blu-ray. Unfortunately this method doesn't yield the best quality results. Your downscaled menus and video will look a little pixelated, and your button highlights will look terrible.
For the best quality, you will want to downscale your menus in Photoshop using the "Bicubic Sharper" algorithm, and you will want to downscale your video in the Adobe Media Encoder with Maximum Render Quality on. If you have a Mercury Playback capable graphics card, you can use Premiere to scale down the video in realtime for even better downscaling quality (as it uses a superior algorithm.
You will then want to start a new project in Encore for the SD version. Sure, this process isn't nearly as fast as simply selecting a different output format in the build tab, but necessary as Adobe has yet to implement a high quality scaling algorithm into Encore.