I was in the same boat a few years back and went to Las Vegas for a work convention and spent an afternoon riding mountain bike trails about a half hour out of the city. It was very scenic, good flowy trails for a roadie getting into mountain biking, and there was a bike shop that had great rentals -- I got an Ellsworth Epiphany for a reasonable day fee.
I can't compare it to many other places, so someone with more experience surely has a more informed view, but in terms of trip planning, LV is usually a pretty cheap and easy destination, and early spring would be a good time to go.