We took a safari trip in St. Lucia last year and had to climb down the side of a good size hill, not an easy climb, to a waterfall. There is a waterfall in Dominica as well.
I've heard there is a small waterfall in san juan. Not sure, but it may be in the same vicinity of the rainforest.
If your doing s.caribbean you'll leave from san juan (maybe you could do a pre or post night trip and check it out)