First of all, make sure each of your children has a back pack. Put a water bottle, snacks, sun screen and a change of clothes in each pack. Label each item. Make the children responsible for keeping it packed with the water filled, the snacks refilled before each trip. I used to make up a peanut butter sandwich to stick inside the b-pack. Each time you are planning on going somewhere, each child should be in charge of their own bag. Eventually they will just do this on their own.
I used to check out books on tape or Cd for the car ride.
On Wed's there is a free event at Symphony Square for children. It is called "Music in the Park". They have story tellers, music, magician's. It is lots of fun. Symphony square is located on Red River in Austin. Check on Statesman.com for more family friendly things.
In the summer we used to "camp out" in the back yard. We purchased a tent from a garage sale and we would set it up every once in a while in our back yard and sleep in it, just like we were camping. We would grill dinner and then breakfast in the morning. It was a good way to get out of the house, but still have all of the amenities.
Get on a city bus and take a picnic lunch.Teach the kids to get on and off of the bus and to pay to ride. There are some days that it is free to ride.
Go to the Capital Grounds and let the kids run up and down the "hills" on the west side of the building. Take stale bread or cereal to feed the squirrels and pigeons.
My daughter loves books so we went to many book store story times. They usually have a story and then an activity. Local libraries also have story times.
On Wed some of the movie theaters have free movies for children. Then they have special snack trays for the kids at special prices.
Look for the free Parenting newspapers and Magazines for the Austin Area. I used to pick them up at libraries or grocery stores. They have tons of free activities.
On our drive way (we do not have sidewalks), we would park our car on the street across the end of the driveway. Then we would take sidewalk chalk and make "roadways". We would do the yellow stripes, parking spots, stop signs, yields.. all the things kids see on the road. Then we would gather up all of our trikes, cars, wagons and let the kids "drive on the road" and have them follow all of the road rules.. It was wonderful to see how they loved making a different map on the driveway each time.
Go to an appliance store and get big appliance boxes. Take them home and let the kids turn them into playhouses or buildings. They can paint them or decorate them with markers. tape them together together and cut doors into them to make a maze... Your 11 year old will love organizing these into different configurations..
There are many things going on all of the time that are family friendly. Just check the notices at the grocery store, church bulletin, statesman.com... You will have a full schedule.