Harpers - lots of water, big water. On the safety side, I prefer to fish Harpers when the Point of Rocks gage,
http://waterdata.usgs.gov/nwis/uv?01638500, is less than 2 feet, 1.75 is even better. If you're younger and bolder than me, maybe 2.25 and dropping will be okay. ;-)
Time spent staring at Bing and Google maps set to aerial view will tell you quite a lot.
Lots of structure between Dam 3 and Byrnes Island - there's also several rubber hatches in the course of the day from the WV side. ;-) And unless you get here early, parking on the MD side can be a bitch. Weverton area is pretty good too, it's the sector just upstream of Knoxville on the MD side. I hear Lander is pretty good (half way between Point of Rocks and Brunswick) but I haven't fished it.
Also: MD license is good throughout this area on the Potomac. But up near the US 340 bridge you need to be aware of being in VA or WV if you don't have a MD license. Then again, I've never been asked to produce my license - could it be DNR or Park Service assumes fly fishermen play by the rules? ;-)
Mark