We've been taking ours to playgrounds in other counties in MD for a month, since those have been open. We just find small neighborhood playgrounds and they are almost always empty. UV does a good job killing virus on surfaces, so let the sun do its job in the morning. I'm a bit leery about them going down the "tunnel" slides (no sun) but I still let them do it. We sanitize hands after.
https://www.montgomeryparks.org/parks-directory/
Use the park finder. So many small parks tucked in a neighborhood which never get visited.
Another idea -- playgrounds at elementary schools. I guess those will be open also?