It's alright for humidity to fluctuate during the day, so just keep up on spraying the moss and substrate as needed. A waterfall can help with a bit of a humidity boost, and if you leave the bottom of the waterfall off, it is actually a nice large water bowl as well. You could also just put a simple water pump or air pump into a big water bowl, and the action of the pump bubbling the water would work much the same. Doesn't look as nice as a waterfall, though.

-Jen