There is a river trail that starts in Irvine that goes down to the back bay in newport & back. It follows a stream, goes by a golf course, etc and is very nice as long as you don't get hit by a golf ball. It is a lot more enjoyable than the SA river trail where the industrial portion is. However, depending on how many miles you want to ride the SART can be very enjoyable. If you park or start up by the Green River entrance off the 91 and rided down to Anaheim stadium and back, its a nice 25+ mile loop. Not the greatest marking is the Yorba Linda part, but still the ride is very nice.