We are going to Downtown Waukesha, I think it will work best if we meet at the Gazebo on the north side of the Fox River on Bank Street between Madison St. and Barstow St. It's across the Street from Waukesha State Bank. You can also see the Clock Tower from the Gazebo. (There's plenty of free parking by the Gazebo and it's a lot less congested than the Steaming Cup area which is only about 1-2 blocks away.)
Here's a link to Google maps
Plein air guide can be found on schedules page