Our first experience with respite was a disaster, in part I think because I didn't know exactly what I needed to ask for and emphasize about her needs. Mom won't complain and can't ask for what she needs and this time I won't be around to supervise, so she is totally at their mercy... I know that sounds awful but I don't know any way to put it.
I think I would bring her favorite things - special blankets, pillows, items of comfort, CD player with favorite CDs, or a radio, books if she's a reader, photos of you and your family to provide a reference point and help her remember that this is respite and not a permanent stay.
Also, perhaps a calendar marking the dates you'll be gone and when you're be returning. A staff member could help her cross off each day. Then she can see when you'll be returning.
I might also write out a lot of cards, one or two for each day, and ask one of the staff to make sure she reads them on a daily basis.
What I've done for family members with special health challenges is get together a batch of cards, mostly animals with different expressions, for different moods. I found "sleepy" cats, energetic cats and dogs, resting wildlife, and others. I used my collection of rubber stamps to add decorations to the envelopes and put suggested moods for each - i.e., when someone is feeling great, feeling poorly, getting restless, tired, wanted to just rest, etc.
So there were cards for a variety of occasions. They could open cards specific to their mood that day.
I'd probably also call, perhaps a few times a day, again as a reference point to help keep her focused.
Another possibility is discussing with the staff what activities she might want to attend and ask them to be sure to help her get to wherever they're being held. That way she won't be alone in her room.