Hwy. 120 to ride into Yosemite Park is a perfectly good bicycling route. But there are some alternative routes to get you off the main road for sections if you prefer. Just outside of Groveland you can use the 'Old 120' route for awhile which runs just south of the current highway. And a little after Buck Meadow you can take Cherry Lake Rd. which takes you to the ridgeline slightly north of Hwy. 120 on the way to Mather Camp before heading back south to rejoin 120 just before the park entrance. That's the route we took on our way back from Yosemite here:
http://www.everytrail.com/view_trip.php?trip_id=1064969
(you can scroll around and zoom in on the map to get details of the route)
If you continue east from Mather Camp you can descend down to the Hetch-Hetchy reservoir.

Wards Ferry Rd. is also recommended as a scenic road that's very lightly traveled. But it is a steep descent down to the river and a long climb back up the other side.