We went to the Wacky Rollers Challenge course (tree top trekking) and were delighted by the whole day. The course was very well designed and laid out and safety was paramount. The staff were at every "game" and made sure everything was done correctly and fun! I have been on other treks, in Canada, and they were not as vigilant.
There was lots of variety in games and many zip lines (my favourite). It also gave you a chance to get a feel for the rain forest and the island (both in the air and while driivng to and from). This excursion was through Princess although I am sure the other lines have similar connections. Highly recommended.
(Pay attention to the weight restriction, one person in our group was not allowed to particpate as he was too heavy. You can also arrange to go as a non participant if you want to go with family but not climb yourself)
Dominica is definitely an eco tour type place, it is not a shopping day. The people we were with were very hospitable and proud of their country (which is gorgeous).
Don't worry about rain in the course, it rains and stops and rains and stops and it is warm (it is a rainforest!)