Sen Monorom is a staging area for treks, or as a home base while visiting the Elephant Valley Project or for riding elephants with local mahouts.
The town itself is pretty dead. There seem to be more cell phone shops than anything else, which completely confounds me. It was interesting to arrive there while the "disagreements" regarding how "best" to deal with an influx of tourists who come to the area to experience elephants. Some local mahouts are opposed to the way the EVP presents the issues; likewise the EVP seems committed to their vision of ecotourism with visitors not riding the elephants. Thus, political and cultural friction. I used the services of both, and am glad I did. As with most issues in our world, there are no simple or either/or answers. I recommend doing what I did: both, as I would not have wanted to have missed either. Do be sure the mahouts you choose are good to their elephants: no heavy chairs for tourists, no ribs showing...and enjoy both experiences! Beautiful area, different topography and vegetation than most of Cambodia.