A.P.
Cedar Park has a great playground/sprinkler park area (free!)
Go north on 183, exit Cypress Creek, go east. follow across Parmer. It'll be on your right, less than a mile after the intersection at Parmer (the road changes from Cypress Creek to Brushy Creek, I believe).
Some other fun places to go are:
Zilker Park - playground and train - from Mopac, head south and follow the signs for Zilker
Radijazz - This is over by Highland Mall. Take 2222 and go east until you see the ACC building. It's in a shopping center on the outside of the mall - close to I-35
Ready, Set, Play - In Round Rock off of exit 251 - It's an inflatable bounce-house place.
CedarRock Railroad - County Park off of 1431 and Sam Bass - great train for kids, park and playscape area.
Children's Museum - downtown at 2nd and Colorado - love love love it! Perfect for hot days!
Try meetup.com - I bet you can find a mom's group that'll be fun of fun ideas!