Was going to add the Willow City Loop and glad "timandsusan" beat me to it. Outstanding drive at any time but "over-the-top" in the spring. About 18 miles north of Fredericksburg on Highway 16 and a bit east on Highway 1323. Follow the signs from there. Be aware this is a private road and we are guests there. No stopping and watch for cattle on the road. Also, watch for cattle grates in the road. Not an issue, just go slow.
Also, Highway 16 between Fredericksburg and Llano, Park Road 4 between Highway 29 and Highway 281 (passing Inks Lake State Park and Longhorn Caverns) and anywhere on 281 between Hico and San Antonio are likely to yield outstanding views of bluebonnets and other wildflowers.
Enjoy and Happy Camping!