I collect buttons, and have some quite old and (relatively) valuable ones, so before I make any suggestions, I wondered; what kind of buttons? Are you interested in preserving the buttons? For example, a random assortment of common mid-20th flat colored buttons I would make different suggestions than a collection of Victorian or Edwardian porcelain and glass buttons.
That said, a resin tabletop or tray, similar to the type a lot of people have done with pennies could be really cool and pretty, but obviously you only want to do that with buttons you have no interest in preserving as buttons,