Have you looked into Endlers Livebearers?
They're smaller than guppies, and their colors are so vibrant - I can't see them not catching her (or anyone's!) eye, and you could keep an AMAZING shoal in a tank this size! Of course you could keep multiple shoals, too - but love nothing better than seeing a big group of small fish. . . I wonder how many you could stock in a 55g tank???!
. . . have you considered shrimp? I wonder if she'd enjoy them? They're busy and bustling, and completely different from fish. :) I don't know much about shrimp - I've never kept them. . . one thing I DO
know is that there ARE yellow ones. . . hopefully someone who knows these things will pop by soon with more information!
I've never kept either of these animals. . . they just popped into my mind when I read your post, so I thought I'd throw the suggestions out there ^__^ Please be sure to look into the needs of both - I'm not really sure if they're ideal for your water parameters (should probably check these things before posting :P)
Oh! And another interesting something-completely-different option to look into might be African Dwarf Frogs. I've kept one in my 55 and he loved it. A nice grouping of them would definitely be something I'd LOVE to see. . .
Good luck in your quest for the perfect stocking for this tank. I'd love to hear how this project progresses - so please keep us up to date on what you ultimately decide on! Whatever you end up with, I hope she adores it! *hugs*