I'm in Austin, and one of the first trips we took last summer was down to PINS. It was the first weekend in September, our camper has a/c but as you know there are no hookups down there. There's always a strong breeze off the Gulf.. I haven't needed a/c at Port Aransas in late April either. We're about to plan another trip to PINS and I doubt I'm about to buy a generator to use the a/c.
Some good spots to go: Pace Bend park west of Austin can be fun for boondocking. There are lots of state parks in Texas, but they book up fast. Private campsites can be a good option, and they tend to be themed.. there are a bunch of private sites along the Guadalupe and Brazos rivers for tubing. This past weekend I went to Hidden Falls Offroad Park with a friend. We 4 wheeled during the day, camped at night. I think that the Louisiana coast is boondockable as well. As for camping music festivals, there is Old Settler's at Camp Ben Muculloch which is in April, and Art Outside at Downtown, Texas in October.
If you want some more details about the camping I've done, feel free to send me a message.
1995 Toyota 4Runner SR5 4WD 5 spd
2002 Eagle Summit 12