I love Berry Springs Park
Berry Springs Park and Preserve It's actually in G'town but just up a few exits from Round Rock but it's worth the drive. There's a pecan orchard there and I find it to be very quiet.
Old Settler's Park has some nice spots but can be busy. It's right behind the Dell Diamond.
City of Round Rock - Old Settlers Park It think that it is large enough for you to find a spot to shoot. I do a lot of shoots there with families.
Memorial Park is also nice (bridge, ducks and not far from the "Round Rock") but can be busy in the evenings.
City of Round Rock - Memorial Park It is a smaller park and right off of 35.