Thanks all.
I ended up getting two of the babals and a Richard Scarry book.
I’lll definitely keep some of the other suggestions in mind for future use. I’m particularly intrigued by the soap crayons and the Crayola crayons w/special paper.
Not to poop on the party, but those crayons can be difficult to remove, depending on your bathroom surfaces (although if it’s entertaining the kid for hours, no sacrifice is too great).
And if you buy that crayola paper, get the blank stuff. The “coloring books” are only treated on selected parts of the page, so that coloring on most of the paper has no effect at all. My daughter liked the fingerpaint, but it’s got a texture like vaseline, kinda icky. Picked up a set for cheap(ish) at Costco, though; $12 for 3 coloring books (one of them blank), fingerpaint & markers.
Oooh, good buy! 
Good point. I have those newer shower/bathtub combos, so they’re all hard plastic and the ‘crayon’ comes off easily. I don’t know what it would do to tile.
Find the largest cardboard box you can. Appliance size is best.
Just give her that.
Best.gift.ever.