One of the many things I really like about San Diego is the canyon parklands which run through the 'burbs (and even quite close to downtown). Although the canyons can be a bit scary at times (like fire season...) they are great for going on a nice hike and feeling like you've gotten out in nature, even though you might be hiking right along a freeway.
Check out the link above for a list of all the canyon parklands and maps of each.
You'd never know we're hiking right next to a freeway!