Wow, bless your BIG heart for taking in and loving such a special one
. I love it! She sounds so precious and sweet. Would love to see a pic of her!
For putting on weight...try feeding some calorie dense treats like a little peanut butter and cheese...that should help a bit. Maybe some yummy doggie treats too, meat based ones.
For enrichment, maybe Kongs, toys that make sounds etc...for example that one toy that has a ball w/ a tail attached to it - it moves around and makes noises.