Leave Fredericksburg about 10 am. Spend about 45 minutes at Old Tunnel State Park (where millions of bats flock out of an abandoned rail tunnel on summer evenings) & about 45 minutes in the small town of Luckenbach. Then take a good 2.5 hours to explore LBJ State Park & National Historic Park (two adjacent separate entities). This is the family and boyhood home of president Lyndon B. Johnson.
Eat a late lunch/early dinner in Johnson City (of course you can cook at your campsite if you wish), then drive to Pedernales Falls State Park & set up camp. Leave time for hiking & swimming, if you wish.