I do agree with Bunny House that you have to careful about pine when it comes to litter and housing. (the phenols need to be removed). . Pine shavings as a litter has not been recommended due to the phenols that can affect the liver negatively. And pine pellets need to be free of the accelerants (if wood stove pellets). But as as far as chewing “untreated” pine, I have never heard of it to be toxic to chew on. The main HRS site, that Bam linked to, includes pine firewood; and untreated fresh pine lumber attached to cage so it doesn’t move–piece of molding, 1″x2″s, or 2″x4″s; as one of their suggestions for chewing. We actually have used that second suggestion of untreated pine lumber to cover hot chew spots and almost all my bunnies in the last decade or so have loved chewing on that.
But if you have any concerns, there are definitely other options as Bam and BH have suggested. Willow, Aspen, Dogwood, Artichoke Sticks, Apple (fully dried). In the BinkyBunny store there is a variety pack of Nibble sticks which offer 4 different types of bunny safe wood chews.