Yeah, Callahan is a good post-work ride if you can't get out somewhere sooner to catch sunlight.
Ashland State Park can be a cool rooty ride if you're in the mood to beat yourself up but still get a good workout. Just get out there and ride around the lake in a big circle. Make sure your shorts have extra padding.
Metrowest surprisingly doesn't have much in terms of riding - you've pretty much hit all the locales in the above few posts.