A.G.
You have to be resourceful RM. Check online, Austin Statesman and other local magazines and newspapers, the library, local churches, the Y, the mall, all the Austin venues, and Austin vistors bureau for free or cheap family friendly events, programs and shows.
There is a state park off 360 that has great hiking trails and its free. There are 4-5 trails ranging from a mile to a few miles roundtrip. We also like going to the reservoir for picnics.
How far are you willing to drive? Austin has TONS of day trip options. You can also get up early, drive to San Antonio and do the mission trail there. That's free.
There is also a ton of stuff you can do at home together. It just takes planning and organizing. Clean out a room and redo it together. Learn a new language. Put on some music and dance. Look up easy science projects online and do one a day. Have 2 kids do something together while you get some 1 on 1 time with the third and rotate the kids each day. Look up the kids eat free schedules in your area. I would also integrate some sort of schoolwork or reading time too, not only to keep their minds sharp before school starts again but its always a great time filler. Get a schedule together so the TV won't be on all the time and noone is wandering around like a zombie.
Last, never underestimate the power of playdates. Totally free!