Brushy Creek Lake Park in Cedar Park is a wonderful park, with something for just about everyone.
Where ever you live in Austin, it is well worth the commute for a full summer day. And one of the best parts is that it is all free!!
The park, acquired in 1987, is 90 acres including a 38 acre lake and has miles of trails for bikers or runners. The trail winds along Brushy Creek Park and then runs past the lake and goes under Palmer lane for another couple of miles (not sure of the distances).
If you head out on a weekend, there is a canoe/kayak rental place for a chance to get in some paddling! Bring bathing suits (for the sprinkler pad, not the lake!), bikes, skateboards, fishing rods and hiking shoes!
The park is split into two, divided by Palmer Road (or maybe it is two parks? Not sure) On the west side are a large number of sports fields and a really fun playground!
Also on the west side of the park is a neat skate park which you can also use for scooters. Not knowing much about skateboards, all I can say is that the variety of ramps, turns and bowls was pretty amazing!
On the east side is the lake with kayak rental, another nice playground, a labyrinth and what I believe is the most fun sprinkler park in Austin (though it does get crowded during the summer!)
If you have a free day in the summer, this is a great place to come and enjoy a huge variety of outdoor recreations!