Household humidifier works great, done it.
Those vaporizers, like when your sick, works great, done it.
In my closet, I got these little gel beads, They are for humidors, They absorb water, 300x thier size, and slowly release. I used a bowl full of those to add about 5-10% in one closet, and 2 bowls to add about 10-15%.