blackwater is pretty good where they stock it, in and around the town of davis and the state park... have tried fishing the upper section at various places in the valley outside of the stocked sections and not had much luck... (unless you find chubs to be lucky)
also, forget off hand which branch and fork of the potomac flows by Seneca Rocks but right behind the national forest visitors center there's some pretty good freestone fishing. The trail to the top is also a great group hike activity and beautiful 30 min drive from canaan. bring your camera!
there are also several tiny brookie streams, and some wild brown and rainbows mixed in from stockings that took place long ago, throughout the area... would recommend scouting a topo map and then checking the interwebs for more info on specific streams since there are several in the area that are dead from mine runoff and insufficient acid rain buffers.
a few of the resorts/communities have tiny holding ponds that are good for sunfish, 10-16" largemouth and occasionally stocked with trout.
a couple other helpful bits...
not sure when you're going but may/june snow isn't unheard of up there.
lot's of bears.
buy food and beer before going. only grocery store is way over priced.