Back in 2015 I received a $25 bank reward for referring a friend to get an account. Years later I discovered that it is considered taxable income. What do I do?

Do not feel guilty that you missed this. The best thing you can do is to file an amended return to report the $25.00 as other income. 

If you already filed your return you will need to wait to see if it was rejected or accepted. If they were rejected you are able to login to your return to fix what you need to fix and fix the errors that caused the rejection and refile the return. If your returns were accepted, then you will need to file an amended return. You should not try to amend your return until the original return it has been fully processed and you have received your refund or your payment has cleared.
Amended returns do not “catch up” to your original return and replace them; they are processed as two separate returns.  
Amended returns have to be printed and filed by mail.  It can take the IRS up to 12 weeks or longer to process them.   If you are due a larger refund than on your original return, your amended return should only show the difference and you will receive a separate check for it.  If you owe money on your amended return, it will show only the new amount owed, you will have to mail a check with the return. You can check the status of your amended return by clicking here: https://www.irs.gov/filing/individuals/amended-returns-form-1040-x-/wheres-my-amended-return-1

 Click here for amending instructions: https://ttlc.intuit.com/replies/3288565

Well what if I just donated $25 to a charity of my choice instead?

Technically you should report it as income on the return and then a charitable donation in the year that you donated it