Turndown service at Disney is actually like a mini-housekeeping. They replenish any towels you may have used in the afternoon (nice for those of us who do come back to the resort for a shower before going back to the parks), they'll remove any room service trays, or any dirty dishes you may have brought back from the club lounge. They'll replenish the toiletries if you used up the last of the shampoo with your afternoon shower. They do pull back the covers to the bed and leave treats associated with the resort. If you have children, they usually have the time to place them in adorable vignettes. You can come back to find them all sitting on the bed, while one of them reads a book, or holding the remote and checking out Disney tv. Very, very cute. They may leave the tv on, or your radio playing softly and one of the bedside lamps on low.
I usually tip per bed, so around $2. My kids have always had fun decorating the tip envelopes, so we do that all in advance of our trip. I put the dollar bills in the envelopes and slip them into the room safe. Each day, I pull out one marked for housekeeping and one marked for turndown. Never had a problem with that system.