The Horse Riding is a Plantation ride - if you are asking about the 'free' horse riding. It's well worth it and we have done it both at Tower Isle and when we stayed as CSS. Will do it again when we go back to TI next year!
For some reason, both times we have gone, it did not seem very busy and it was only our party of 4, which we rather enjoyed.
One word of warning - the riding helmets REEK of othere people's 'sweat', so suggest you take either a bandana, or do what I did - kept my baseball hat on and put the helmet on top.....but then again, I am very fussy!!!
The 8 am horseback riding at CTI was along the white river and then up a mountain / big hill. What made it nice was learning about the vegetation and some history of Jamaica and all the jokes by the guides.