The big name streams of the Cumberland Valley are your best bet: Letort, Falling Springs, Big Spring. Gunpowder in MD would also be a good bet.
Breeches can fish well in summer, but be careful with water temps. Clarks Creek can fish well too and should be colder, but it may be very low, but worth checking out. There are other spots a bit off the beaten path, but these are the big names.
Do some exploring and take your stream thermometer with you.
Good luck.