We'll be at both locations this winter. In Dominica we found this place and made hiking reservations. Lots to choose from.
In Grenada we will be taking a private hiking tour of Concord Falls with Mandoo Tours. They charge $25 per hour. Our first time there we went with Pete's Mystique Tours and did the Seven Sisters Falls hike...about a 45-minute hike through rainforest and very nice. Stopped to buy some spices along the way. The roads on Grenada are winding, steep and downright scary in spots. Lots of poor folks...very sad.