We just visited the Mission Creek Preserve in the Desert Hot Springs area of the valley. This is a beautiful area and off the beaten path. You can get an idea about it from our photo album. They also have a website you can find if you google. It is maintained by an organization called the Wildlands Conservancy. Hope you enjoy your visit. Let me know if you would like more ideas.
This is Trixie (from the Coachella Valley group). Where do your parents live exactly? When in April will you be visiting? How old are your kid(s)? I'd be glad to help you find some "earth-friendly" activities but I do have to warn you it may be starting to warm up by then (especially for those not used to the heat). I look forward to hearing from you.