generally apple/rose family fruit wood (apple quince pear rose) is safe. People prune those in the winter, so they are nice to gather and keep. alder, fir are also fine for them to chew. for all other questions I recommend searching "_plant name_ toxicity ". Also if a particular ornamental is listed as "deer resistant" it is usually not good rabbit food either.
That said, "boxelder" is a common name, be sure you are identifying the correct tree by comparing pics and checking the latin species name for toxicity also.