Idaho voters can register at the polls on election day. From the Voter Registration website:

You may register at the polls on election day by providing proof of residence. All documents used in providing proof of residence shall be accompanied with a photo I.D. Only the following documents showing the registrant's current address in the precinct are authorized:

• a valid Idaho driver's license issued through the department of transportation
• a valid Idaho identification card issued through the department of transportation
• any document which contains a valid address in the precinct together with a picture identification card
• Students may also use a current valid student identification card from a post secondary educational institution in Idaho accompanied with a current student fee statement that contains the student's valid address in the precinct together with a picture identification card.