As mentioned by others, Double Rock, North point, and Fort Howard are all great and I'd also add Leakin Park to that list. If you want to venture out a little further Annapolis Rocks is a great day hike near Frederick. Further out still is Old Rag Mountain which is in the Shenandoah Valley and is one of my favorite day hikes. This book has lots of good suggestions as well: 60 Hikes Within 60 Miles: Baltimore