(Nerd alert!)
A dice roller
. Almost every game book comes in large format and color, PDF for the ebook form.
(Nerd mode off)
Keep the regular old calculator which I assume it has now.
I'd also like some sort of in-page placeholder to mark where you are in complex instructions for hobby books. Finer than a page-based bookmark and easier to move than having to highlight and unhighlight (plus highlights won't work well for graphics and perhaps not in sidebars.) Maybe a ruler line across the page or something. This might already be there for note-taking academics.