I was reading all about Horsetooth Res and the water sports access sounds great, but is there a place nearby that you can ride ATV's? Perhaps a rental place as well?
Sorry for the bad news. The Fort Collins area is populated mostly with bycyclist. Few ATV's.
On the other-hand, Wyoming has more ATV's than most states.
All ATV lovers probably should re-locate there.
Fort Collins is not very outdoorsey.