Yosemite by Bike
I spent part of last week in Yosemite National Park, and one of my favorite finds was renting bikes in Yosemite Valley. The valley's few roads were clogged with cars, even on a weekday in mid-May, but the bike paths were blissfully empty. You can navigate along these paved trails to a number of the valley's best spots--Mirror Lake, the trailhead to Vernal and Nevada falls, the swinging bridge over the Merced River. You can also take the bikes onto the valley's roads, though be aware that there aren't any shoulders and you'll be competing for space with SUVs and tour buses. Parking your car is a nightmare anywhere in the valley, but there's always a bike rack nearby (and you don't even have to lock up your rental). With all these reasons to rent a bike, need I add that it'll save some fossil fuels, too?
My only disappointment was seeing how few bikers wore helmets. They're free with the bike rental, and well worth the bad hair day. Don't think you can do without one just because you're riding only on the trails, away from vehicular traffic--one trip over the handlebars after stopping short for a scurrying animal and you'll be happy that dent is in your helmet, not your head.
You can rent bikes at Yosemite Lodge (also a great option for overnighting in the valley) or Curry Village.