There are a number of easy ways you can contribute to keeping the ports tree up to date and in good working order:
Find some cool or useful software and create a port for it.
There are a large number of ports that have no maintainer. Become a maintainer and adopt a port.
If you have created or adopted a port, be aware of what you need to do as a maintainer.
When you are looking for a quick challenge you could fix a bug or a broken port.