If you want to get a little north of the city, I highly recommend Germantown Dam Park http://www.metroparks.org/Parks/Germantown/Home.aspx
It offers some excellent hiking trails and very nice scenery. Plus, the section of Twin Creek that runs through the park offers some pretty decent small stream fishing.
Montgomery County MetroParks has also constructed a trail linking Germantown Dam Park with Twin Creek Park http://www.metroparks.org/Parks/TwinCreek/Home.aspx
Twin Creek is the site of an old Boy Scout camp where my troop used to do a lot of its' activities. It's kinda strange for me, being there. There are some abandoned buildings there and the ground just holds quite a few memories for me.
Anyways, these two parks (especially Germantown) are worth finding and taking time to explore.