It sounds like they are in a standard air-conditioned room. You should have no problem getting the humidity down by adding ventilation then. I think removing materials that retain water will help but you may find that the humidity becomes less stable because the amount of water that is exchanged with the outside environment each hour will be much larger relative to the total amount of water inside the enclosure. Conversely, larger amounts of these materials will make it more stable but then you will be forced to ventilate more to keep the humidity down. And, just to keep things exciting for you, additional ventilation will most likely make the temperatures less stable. If I've managed to make any sense here you'll see that there are some competing factors at play here and a bit of experimentation will probably be required to reach a happy medium.
Good luck.