There are many event management plugins and volunteer management plugins available for WordPress, each with its own strengths and weaknesses. Some of the best WordPress event plugins for non-profits are:
Events Manager: A free plugin with a Pro version that offers premium support, custom payment gateway, API, spam protection, coupons, discounts, customizable booking forms, and more.
WP Charitable: A popular donation plugin that lets create unlimited fundraising campaigns and customize them to the needs. It supports leading payment processors and does not charge any transaction fees. It also has a premium version that offers recurring donations, crowdfunding, peer-to-peer fundraising, and more.
The Events Calendar: A free plugin with a Pro version that offers advanced features like recurring events, community event submissions, Facebook event syncing, and more.