You can grab a tube of Nutrical or Nutristat at any pet shop. If they don't have those exact brands, any high calorie food supplement will do. You can just mix a bit of it into the cereal you're giving her. It'll give her a few more vitamins and minerals not in Ensure.
You can also try pureeing some regular foods for her. You can crush up some blocks and mix it with ensure, toss some pureed veggies in.. You can get some small jars of baby food and mix those up with any of that, including ensure.. You can put most of those into a shallow bowl, or maybe syringe it if it's liquidy enough..
As Jo said, the Ensure is the best thing for her, but if you want to try mixing some things in with it, that should work, too.
