Try to aim for regular volunteering: a few hours of a week looks good and you get more from the opportunity. If you are able to volunteer in a position of responsibility, this can look good to.
There are lots of different types of volunteering depending on your interests, e.g. fundraising, after-school clubs, charity shop, conservation projects, etc. I would recommend going to your local volunteer centre or looking at do-it.org to see what is available in your area.