Remind your tenants that their LEASE specifies that they cannot put holes into the walls of their rental. Give them a flyer or send them a flyer with a coupon telling them about wall savers!
The people at 3M (Command Products) have developed a self-adhesive product that allows you to hang items of various heights and weights on the walls. This product can be removed after use without any damage to the walls.
You can download coupons from the internet to give to tenants as an incentive.