She decided to volunteer at the animal shelter on weekends.

To offer oneself for a service or undertaking without being paid.
I will volunteer for the task.
They volunteer their time often.
He wants to volunteer abroad.
volunteer work
volunteer to help
volunteer for a cause
offer
serve
assist
She is a volunteer at the local animal shelter.

A person who offers to take part in an activity or service voluntarily, often without pay.
He is a volunteer firefighter.
She became a volunteer tutor.
Many volunteers helped the cause.
a volunteer organization
a volunteer position
volunteer work
helper
participant
contributor