How do I unlock an old iPad with Activation Lock without knowing the Apple ID password?
Apple has the following information on their page for Activation Lock for iPhone, iPad, and iPod touch:


  If you forget your Apple ID or password Your Apple ID is the account
  that you use to sign in to all Apple services. Usually, your Apple ID
  is your email address. If you forget your Apple ID, we can help you
  find it.
  
  If you forget your password, you can reset it from your device or your
  Apple ID account page.
  
  If you can't find your Apple ID or reset your password, you'll lose
  access to your account and might be unable to use or reactivate your
  device. To help prevent this, visit your Apple ID account page
  periodically to review and update your account information.


Unfortunately, you are probably out of luck.

Since your iPad was unused and running iOS 9, it's probably pretty old and may not be worth any money in Apple's Trade-in Program as a non-functioning device, but can be recycled free of charge. (Nation-specific recycling programs can be found here). Other people or companies may be willing to purchase non-functioning devices for parts.