We travel a lot in the summer too, with our 2 boys. Ours tell us when they need to take a break, usually about every 2 hours.
One thing to be careful of is when you stop and have them out on the leash to potty, watch the ground and make sure they don't eat something they shouldn't. Gas stations and truck stops, etc. have lots of trash/cigarettes/food around the perimeter and I make sure my boys don't eat something on the ground.
We usually carry an ice chest and after we stop for a potty break, they are very hot, so I put some ice into their water (I carry their water in a plastic container with a lid) so that the water is cool/cold. That really helps them to cool off.
We eat at drive-throughs or one of us goes in and orders and the other stays in the car with the boys and then we eat in the car. We have been known to park the car right by the restaurant window, leave it running, go inside and sit right by the window and eat.
Always know where the leashes are in the car, make sure they are within easy reach so that you can get them leashed up in case of emergency.
Now! For Austin - it's HOT here! To cool off, take a dip in Barton Springs pool.
Barton Springs, Austin - Things to Do - VirtualTourist it's 68 degrees year round.
Here is a list of the dog friendly places in Austin, and it is not a complete list, but it's a start.
DogFriendly.com's City Guide: Austin, Texas
Austin is a very dog friendly city and you will find tons of things to do with them - but it is very hot right now. It's best to wait till fall (October) to get them out and about very much. A fun place to visit is Lofty Dog on 2nd street
Welcome to Austin Lofty Dog! The owner is great and they have lots of fun activities and events. We had a Yorkie meetup there a couple years ago.
Of course, you must visit the Oasis at Lake Travis (not dog friendly) - the food isn't the best and it is expensive, but you can go in the evening (to watch the sun set) and have a beer/drink/cola. It's pretty and fun - but HOT right now. Lake Travis itself is great fun - try Carlos and Charlies.
The Hike and Bike trail around Lady Bird Lake is always dog friendly and a really nice place to get out and about.
You can float the river in San Marcos or further down the road you can float the Guadalupe near New Braunfels. Lots of fun and the water is always very cool.
You will find all manner of things to do here, with and without your babies.
Enjoy!