As Amethyst said, wood shelves, toys, and other accessories are great so long as they're made from a chew-safe wood and don't use any toxic adhesives in the construction.
My girl's cage has a bunch of shelves and accessories that are mostly made from kiln-dried pine, which seems to be the most common wood you'll find for those sorts of things. It's safe, it's relatively inexpensive, and it looks pretty and smells nice. Poplar is also pretty common, and I've come across a few things made from aspen. All three are good and chin-safe, in addition to all those on Amethyst's list. Kiln-dried pine, poplar, and aspen just seem to be the most readily available.
Thanks for posting that list, Amethyst! I was actually looking for it recently and was having trouble finding it.