Austin has some wonderful experiences for families. 

OUTDOORS: 

The first among them being the Bat Colony under the Ann Richards Congress Avenue bridge.  Children and adults alike enjoy the thrill of watching 1.5 million bats make their nightly sojourn into the sky to eat 10-30,000 pounds of insects each night.  This experience is free and Austinites routinely set up blankets and picnics on the Austin American Statesman's back lawn for the nightly viewing.  The bats arrive in mid-March and return to Mexico in early November.

Need a family activity that's fun for young adults and teens?  SegCity Segway Tours on 6th Street - the Heart of Downtown Austin's entertainment center - offers an exhilarating, memorable, and educational experience. Located just a one or two-block walk from most downtown hotels, SegCity Segway  tours run year-round, and there are several departures daily.

Zilker Zephyr is a great way to see the Ziker Metropolitan Park . The prices are 1.75 for children and 2.75 for adults. Children under one are free. The hours of operation are 10-6 pm Tuesday-Sunday but can change,they usually pick a maintenance day during the week. On holidays make sure you check the signs posted before you tell your munchkins about the train!!  Bring suits & swim in the natural springs Barton Springs pool.  In summer, local radio station KGSR hosts free music at Zilker Park & at local restaurants.  There are also free concerts/plays at Zilker (http://www.ci.austin.tx.us/zilker/).

Austin Nature and Science Center has a great little zoo area with local animals and a water area (creek) that they are in the process of beautifying.  The highlight is definitely the dinosaur area.  Plenty of shovels and brushes to "dig" for dinosaur bones.  For more information, visit the website -- http://www.ci.austin.tx.us/ansc/prog.  Austin zoo is another good stop http://www.austinzoo.org/.

The Austin Hike & Bike Trail is really nice.  On Lady Bird lake, you can rent kayaks and canoes near the Hike and Bike Trail.  Check out one of Austin's Frisbee golf courses http://www.ci.austin.tx.us/parks/disk.  Pease Park is the location closest to downtown.

Texas Parks & Wildlife has a lot of links to parks in and around Austin.  In addition to campsites, they also rent out cabins at very reasonable rates (www.tpwd.state.tx.us) - Austin is in region 6.  About 3 hours north of Austin, there is the Dinosaur Valley State Park in Glen Rose, which is terrific fun & educational for little ones.

The Hill Country is surrounded by lakes & rivers and has lots of natural swimming holes, which are a fun way to cool down on a blazing hot day.  This list also includes info on Austin city pools http://www.texasoutside.com/austinfra...  The Deep Eddy pool near downtown is one of the prettiest manmade pools around with large, old trees.  You can rent boats of all types in lakes around Austin, but check the location - things are spread out in Texas!  Volente Beach on Lake Travis near Austin.  Close to Austin are 2 great waterparks http://www.volentebeach.com/  http://www.schlitterbahn.com/nb/ (one of best in the US).  Also, inner tubing down the cool Comal or Guadelupe river is lots of fun & a very Texas thing to do. 

Peter Pan mini-golf, is close to town & always a hit with the young'uns:  1207 Barton Springs Rd

INDOORS:

For a rainy (or too hot) day, check out Austin Children's Museum which is geared toward the younger kids http://www.austinkids.org/ or downtown, the State Capitol (wear cool clothing, parking is at San Jacinto & 14th), the Bob Bullock Texas History Museum.  Older kids may enjoy the LBJ State Library next to UT on Red River or the Blanton Art Museum at 19th (MLK) and Colorado.

For older children, check out a UT Austin baseball, volleyball, basketball, swim meet, tennis match or soccer game.  Football games are fun, but the tickets are relatively expensive.

Take Note

Austin is crowded during South by Southwest (SXSW), Austin City Limits Music Festival (ACL), Texas Relays, and University Interscholastic League (UIL) Athletic events, so unless you're coming specifically for these events, avoid those dates if possible.  Don't recommend 6th Street with little ones on weekend nights because it gets very rowdy.  Ditto for Halloween, Mardi Gras, etc. 

CARS: Austin likes to shut down streets at the drop of a hat (or marathon), so look at the road closure schedule  http://www.ci.austin.tx.us/specialeve...  and Texas Dept. of Transportation (TXDOT) has a handy "Highway Condition Reporting System" that is updated regularly http://www.txdot.gov/travel/road_cond...

FOOD:  It's fun to eat outside - be sure to sample Austin's vibrant food trailer scene (multiple trailers offer something for everyone on S. Congress & S. 1st ) http://maps.google.com/maps/ms?ie=UTF...

SPECIAL EVENTS: 

Feb/March:  Star of Texas Rodeo (probably need a rent car) http://www.rodeoaustin.com/   There is a carnival & petting zoo.

May/Sept:  Pecan Street Festival - art booths, food, free music

Christmas:  Zilker Park Christmas lights (be prepared for traffic), 34th St @ Guadalupe - 2 blocks of insanely creative lights - get your coat & hot chocolate & walk through to get the full experience.

Really good sources of event info:  http://www.ci.austin.tx.us/specialeve...  www.austin360.com &  www.austinchronicle.com - be sure to pick up a free Chronicle newspaper when you're here.