Definitely a drawing or painting - even if you aren't an "artist" it will mean more to her than anything store-bought with no thought behind it.

Or, maybe you could buy an inexpensive frame at a dollar store, and decorate it with her favorite colors, etc., and put "photo of your choice to come" in the space ...

Or make a collage to put in the frame of you, all your pets - those she loves most in the world, and if you are feeling creative, write a little poem to go with it ...

Whatever you give her, it will mean the most because it comes from you.