My fiancee and I are going out to San Francisco in the middle of March to visit her brother Brandon. While we're out there, we'll be driving around and doing the general touristy stuff but we've also set aside a day to visit Yosemite. I know that March is still considered winter out there and since we'll only be there for a day, our options are pretty limited for good day hikes. Do you know anything about what would be good trails for us (nothing too strenuous because we won't really have any gear besides a daypack and I don't think Kelly is up for a Grand Canyon repeat yet).

From the research I've done, Yosemite Valley seems like the best area to stay in and I don't think it will be too busy since its still winter. I was thinking Vernal and Nevada Falls as well as Bridalveil Falls. The Valley Floor Loop looks like a really good choice as well. Any thoughts?

Yeah the valley floor is definitely your best bet. You can see El Capitan, Half Dome, and Yosemite Falls. If the weather is permitting which as of right now it looks like it will be (only one real storm this winter) you can hike up to Nevada, Vernal Falls, and Mist Trail. That is perfect for a day.