One day/night at Garner!!
Not near enough time to really enjoy the place but take what you can get...The river is the prettiest in Texas so Garner will be crowded. You might drive downriver to Concan where Neal's Lodges are and rent tubes and get a shuttle there for a 2-3 hour float trip depending on how the river's running. (Neal's has been named one of the top ten swimming holes in Texas). If you like hiking, do the White Cave trail (in Garner) that takes you up and over the backside of the Old Garner Section (trailhead found near the shelters) to near the top of Old Baldy. Awesome views of the park and river from there. Easy day hike, about 2-3 hours if you don't rush too much. Just take plenty of water and watch your step! And a flashlight in case you are able to sneak a peak into the cave. (It's a popular destination for kids and such. Bop anyone you see that has a can of spray paint
Lost Maples is a quiet park, much smaller that Garner but it doesn't have the swimming that is available at Garner. It does have The Ponds but if swimming is what you want , stay near the Frio. If hiking is more your thing, Lost Maples is definitely worth the trip (about 30 or so miles away). Though you may be tempted to do the East Trail for the overlook view, I find Mystic Canyon on the West Trail a nicer hike. If you do the East Trail, begin the hike via the Maple Trail and be exceedingly careful on the way down as you descend to The Ponds. Again carry lots of water. I've never been there during the summer but I do know how warm it can get during the early spring. We carry at least 2 liters for each of us then. You might need more in June.
Have fun!!